Results of delayed triage by HPV testing and cytology in the Norwegian Cervical Cancer Screening Programme

Abstract

Background. High-risk human papilloma virus (hrHPV) testing was added to the cytology triage of women with equivocal screening smears in the Norwegian programme for cervical cancer screening in 2005. In this population-based observational before and after study we assessed the effect of changing the screening algorithm.

Material and methods. In periods before and after the change 75 852 and 66 616 women, respectively, were eligible for triage, i.e. they had smear results of unsatisfactory, atypical squamous cells of undetermined significance (ASC-US), or low-grade squamous intraepithelial lesion (LSIL) at routine screening. The triage was delayed as supplementary testing started six months after the initial screening. The groups were compared with respect to results of triage and later three-year cumulative incidence of cervical intraepithelial neoplasia grade 2 or worse (CIN2+).

Results. Before and after the change in the screening algorithm 5.2% (3964/75 852) and 8.1% (5417/66 616) of women, respectively, were referred to colposcopy. Among women referred to colposcopy cumulative incidence of CIN2+ (positive predictive value of referral) increased from 42.0% [95% confidence interval (CI): 40.3 − 43.7%] in the period with cytology only to 48.0% (95% CI 46.6 − 49.4%) after the start of HPV testing. For women recalled to ordinary screening the three-year cumulative incidence decreased from 2.7% (95% CI 2.5 − 2.9%) to 1.0% (95% CI 0.9 − 1.2%) during the same period. Among women with LSIL at routine screening and HPV testing in triage, 52.5% (1976/3766) were HPV positive.

Conclusion. The new algorithm with HPV testing implemented in 2005 resulted in an increased rate of referral to colposcopy, but in a better risk stratification with respect to precancerous disease.

Cervical cancer screening is recommended by several international organisations [1,2]. Guidelines on screening policy and organisation of the programmes, as well as monitoring and evaluation, are provided both on international and national level [3–5]. Screening should take place within an organised programme and activities in all parts of the programme should be executed in accordance with given guidelines. The screening methods should be based on scientific evidence, and all aspects of the programme should undergo rigorous quality assurance. There should also be a plan for monitoring the screening process and for publication of the results [6]. Special care is needed if screening methods are altered over time. Changes will usually be based on international research results, but any such changes may require modification in order to be implanted in a local setting. When new modalities have been introduced, their effectiveness should be assessed [3].

Cervical cytology remains the standard test for cervical cancer screening in many countries. Women with negative tests are recommended to have a new test within 3–5 years [3,7]. Women with cytological results indicating high-grade lesions are referred to diagnostic work-up, such as colposcopy. For a long time there has been a discussion on the best management of women with lower grade cytology results, such as atypical squamous cells of undetermined significance (ASC-US) and low-grade squamous intraepithelial lesion (LSIL) [3,8]. In previous decades, the strategy in Norway and elsewhere for handling these women has been repeat cytology (Figure 1). After the introduction of commercial high-risk human papilloma virus (hrHPV) tests, several studies have investigated the performance of hrHPV tests in the management of these women [9–12] and recommendations on their use have been given [3]. So far, there seems to be a lack of reports with results from the introduction of HPV test for this purpose in routine screening. However, preparatory work has been reported [13,14].

Figure 1. Flowchart for triage with repeat cytology.

In the Norwegian Cervical Cancer Screening Programme (NCCSP) the use of hrHPV tests in triage of abnormal cytological results has been advocated since July 2005 [15]. Before that time repeat cytology was the only option. The national recommendations from 2005 include certain options not often found in other studies and programmes for cervical cancer screening; the secondary testing with hrHPV tests occurs six months after the initial Pap smear (delayed triage), and includes a new cytology as well (Figure 2). Another special feature was the triage of women with an unsatisfactory screening smear that took place until 2009.

Figure 2. Flowchart for triage with hrHPV test and cytology. NEG, negative; POS, positive.

The aim of this observational study was to analyse the effects of introducing HPV testing in triage in the NCCSP in 2005. Specifically, we determined the effect of the changes in guidelines on the proportion of women referred to colposcopy, as well as cancer precursor rates in groups defined by the screening process.

Material and methods

Cervical cancer screening in Norway

Norwegian health authorities recommend women 25–69 years old to have a screening smear every three years. A national screening programme against cervical cancer was established in 1995 and relies on centralised registration of cytological smears from the cervix starting in 1991. Pap smears are obtained by general practitioners (GP) or gynaecologists. By linkage to the national population registry the Secretariat of the NCCSP at the Cancer Registry of Norway (CRN) can identify women who are not registered with a smear for the last three years. These women are sent a reminder (personal letter) recommending that they contact their GP or gynaecologist for having a smear. A second reminder is sent after an additional year if no test result is registered in the meantime. The registries of individual screening data are linked to the national cancer registry, which includes information on incident cancers and precursors lesions since 1953. More details on cervical cancer screening in Norway have been reported elsewhere [16,17].

Data sources

All private as well as public pathology and microbiology laboratories are required by law to report cervical test results to the CRN. The reports include personal identifiable information (including the unique personal identification number assigned to all residents of Norway) on all individuals with Pap smears as well as histology specimens and HPV tests. Since 2005 cytological results are coded according to the Bethesda 2001 classification [18]. For this analysis results recorded before 2005 were recoded according to the same classification to make comparable groups. Cervical intraepithelial neoplasia (CIN) is classified according to international recommendations [2].

Triage protocols

Women with the screening cytology results, unsatisfactory, ASC-US and LSIL, undergo additional testing (triage). Before 2005, triage was based on repeated testing with cytological smears. Guidelines at that time advised a new cytology six months after the equivocal initial smear (delayed triage) (Figure 1). If the first triage cytology did not give a clear result (i.e. unsatisfactory, ASC-US or LSIL), a second triage cytology was performed 12 months after the first triage cytology (18 months after the initial smear). Women with a normal result on the first or second triage cytology were advised to have a new screening cytology in three years. The others were referred to colposcopy.

Since July 2005 onwards, the guidelines [15] specify the use of hrHPV tests in triage (Figure 2). According to the guidelines, a HPV test and a new cytology should be obtained six months after the initial cytology. Women with high-grade cytology or ASC-US/LSIL cytology and positive HPV test at six months are referred to colposcopy. Women with other cytology results and negative HPV test at this point are returned to routine screening. If the HPV test is positive but the second cytology is normal or unsatisfactory, then a new HPV test is recommended after another six-month period (12 months after the initial smear) (Figure 2). If the second HPV test is negative, the women are returned to routine screening, otherwise they are referred to colposcopy.

Liquid-based cytology was rare during the study period and HPV tests were taken on separate samples.

Study population

In this observational study we compared the results from triage in the national programme during two time periods. The first period represented the historical comparison group, where we included all women who underwent triage resulting from an initial cytology obtained between 1 January 1999 and 31 December 2001 (Period 1). In the second period we included all women who underwent triage with an initial cytology between 1 July 2004 and 30 June 2008 (Period 2), the group will cover all triage procedures the first three years of HPV testing. In addition, Period 2 was divided into those who underwent triage with HPV test and cytology (HPV group) and those who had no HPV test, i.e. where the physician presumably had continued using the old guidelines (Cytology group).

Cytological smears and HPV tests may be taken for multiple purposes and the registry does not contain information on the indication of each test. Our main interest was triage following an initial cytology obtained as part of screening, or as part of primary prevention of cervical cancer. Therefore, we excluded women with a previous diagnosis of gynaecological cancer and women who within two years before the potential initial cytology had either had an abnormal cytology or had been diagnosed with CIN2 or worse (CIN2+).

The aim of the triage is to separate the women into two groups, one for diagnostic work-up (referral group) and the other for routine screening (screening group). The separation should take place within the time limits of the guidelines. We applied a liberal view when setting these time limits for this analysis. For the triage tests with a recommended interval of six months, we accepted intervals from 1 to 12 months. For the second triage cytology in Period 1 with a recommended interval of 12 months after the first triage cytology, we accepted intervals of 1–18 months. If a HPV test was used for triage, there should be a simultaneous cytology. A smear registered the same month or 1–2 months after the HPV test was considered as part of the secondary testing. Even with this liberal interpretation of the guidelines there were some deviations in testing practice. This was usually because the recommended triage test took place after our liberal upper time limit. For these cases we assigned the date of the upper limit as the end of triage and coded the triage as incomplete. Some events interrupted the triage process; women were censored at death, emigration, or if she had a CIN2 + or gynaecological cancer registered before the appropriate triage tests were obtained. These cases were coded as censored in the presentations of the results.

Five different hrHPV tests were used during the first three-year period of cotesting in triage and included in this study. The three most commonly used tests were the Hybrid Capture II assay, [(HC II, Qiagen, Gaitersburg, MD USA), PreTect HPV-Proofer (NorChip^®, Klokkarstua, Norway) and the Amplicor HPV Test (Roche Diagnostics^®, Basel, Switzerland)]. A comparative effectiveness study of the three tests has been performed [19].

Statistical analyses

For evaluating the introduction of HPV tests in triage two types of comparisons were made. First, the results in Period 1 were used as a historical comparison (baseline) for results in Period 2. Secondly, the results from the HPV group were compared to those in the Cytology group (both within Period 2). These groups were compared with respect to the results of the triage process. Specifically, the groups and subgroups defined by the triage were followed for incidence of CIN2+. Maximum follow-up was to the end of 2003 for women in Period 1 and to the end of 2009 for women in Period 2. Accordingly, the maximal length of follow-up was between 2.0 and 5.0 years for women in Period 1 and between 1.5 and 5.5 years for women in Period 2. Each woman was followed until the end of follow-up, CIN2 + or a censoring event (death, emigration or a gynaecological cancer), whichever came first. Incidence of disease was measured by the cumulative hazard rate and estimated by the Nelson-Aalen estimator [20]. For presentation, we chose to use cumulative incidence computed by 1-exp(-H), where H is the cumulative hazard rate.

Only data on women in the referral group and the screening group were included for the summary presentation of the follow-up after triage (Table II). Outcome measures included percentage referred to colposcopy among those with completed triage, and cumulative incidence of CIN2 + at three years of follow-up for the referral group and for the screening group. The end point at three years was chosen since this is the recommended screening interval in Norway. The cumulative incidence of the referral group might also be called the positive predictive value (PPV) of referral and the cumulative incidence of the screening group equals one minus the negative predictive value (1-NPV) of referral. We also computed supplementary measures that could be called algorithm sensitivity and specificity, i.e. estimates for the proportion of women with CIN2 + that were selected to the referral group and the proportion of women without CIN2 + that were selected to the screening group (Table II).

Analyses were made for all women in triage and separately for each group with respect to results of the initial cytology. Frequencies were compared by test for proportions in two independent groups. Differences between cumulative incidences at three years were tested by estimates from Stata of cumulative hazards. Differences between sensitivities were tested using standard errors for a linear approximation to this parameter.

The statistical packages Stata 11 and Stata 12 were used for the analyses [21]. We used two-sided tests for statistical significance and a significance level of 0.05.

Results

There were 75 852 women eligible for triage in Period 1 and 66 616 women in Period 2. In both periods, more than 50% of the women had unsatisfactory as initial cytology result. The ratio of women with LSIL versus ASC-US cytology was higher in Period 2 (8812/22 224) than in Period 1 (6709/19 987). For the two groups in Period 2, initial cytologies of ASC-US and LSIL were more common in the HPV group than in the Cytology group (Table I).

Table I. Results of triage (%) by initial cytology.

	Result of triage	Initial cytology			Total
		Unsatisfactory	ASC-US	LSIL
Period 1*	Screening	31 491 (64.1)	12 397 (62.0)	3692 (55.0)	47 580 (62.7)
	Referral	1108 (2.3)	1696 (8.5)	1160 (17.3)	3964 (5.2)
	Incomplete triage‖	16 322 (33.2)	5601 (28.0)	1606 (23.9)	23 529 (31.0)
	Censored^¶	235 (0.5)	293 (1.5)	251 (3.7)	779 (1.0)
	Total	49 156 (100.0)	19 987 (100.0)	6709 (100.0)	75 852 (100.0)
Period 2^†	Screening	20 441 (57.5)	13 587 (61.1)	3980 (45.2)	38 008 (57.1)
	Referral	700 (2.0)	2617 (11.8)	2100 (23.8)	5417 (8.1)
	Incomplete triage	14 242 (40.0)	5537 (24.9)	2364 (26.8)	22 143 (33.2)
	Censored	197 (0.6)	483 (2.2)	368 (4.2)	1048 (1.6)
	Total	35 580 (100.0)	22 224 (100.0)	8812 (100.0)	66 616 (100.0)
HPV group^‡	Screening	2288 (85.0)	5892 (67.9)	1759 (46.7)	9939 (65.7)
	Referral	195 (7.2)	1866 (21.5)	1526 (40.5)	3587 (23.7)
	Incomplete triage	202 (7.5)	840 (9.7)	435 (11.6)	1477 (9.8)
	Censored	8 (0.3)	76 (0.9)	46 (1.2)	130 (0.9)
	Total	2693 (100.0)	8674 (100.0)	3766 (100.0)	15 133 (100.0)
Cytology group^§	Screening	18 153 (55.2)	7695 (56.8)	2221 (44.0)	28 069 (54.5)
	Referral	505 (1.5)	751 (5.5)	574 (11.4)	1830 (3.6)
	Incomplete triage	14 040 (42.7)	4697 (34.7)	1929 (38.2)	20 666 (40.1)
	Censored	189 (0.6)	407 (3.0)	322 (6.4)	918 (1.8)
	Total	32 887 (100.0)	13 550 (100.0)	5046 (100.0)	51 483 (100.0)

*Initial cytology 1 January 1999 to 31 December 2001; ^†Initial cytology 1 July 2004 to 30 June 2008; ^‡HPV test for triage (Period 2); ^§No HPV test for triage (Period 2); ^‖Incorrect tests or timing of tests; ^¶Death, emigration, gynaecological cancer or CIN2 + before completed triage testing.

From Period 1 to 2 the percentage of women referred to colposcopy increased from 5.2% to 8.1% (Table I) (p < 0.001). For those with initial cytology ASC-US the percentage increased from 8.5% to 11.8% (p < 0.001) and from 17.3% to 23.8% (p < 0.001) for those with LSIL. In both periods the lower percentage for referral was observed among those with an unsatisfactory initial cytology (Table I). The percentage of incomplete triage was 31.0% in Period 1 and 33.2% in Period 2. In both periods the higher percentage was observed in the group with unsatisfactory initial smears. The percentage of being censored before end of triage was 1.0% and 1.6% in Period 1 and Period 2, respectively (Table I).

A higher percentage of incomplete triage was observed in the Cytology group than in the HPV group, 40.1% versus 9.8%. For all categories of initial cytology the percentage of referral was higher for the HPV group than for the Cytology group.

During follow-up, 2513 CIN2 + were observed among triaged women in Period 1 (Table II), of these 647 CIN2 and 63 cancers. The corresponding numbers for Period 2 were 2763 CIN2+, 998 CIN2 and 38 cancers. Among women in the screening groups, the cumulative incidence of CIN2 + after three years decreased from 2.7% to 1.0% from Period 1 to Period 2 (p < 0.001) (Table II). For the referral groups there was an increase in PPV from 42.0% to 48.0% (p < 0.001). A higher proportion of those diagnosed with CIN2 + within three years was triaged into the referral group in Period 2 than in Period 1. This was reflected in the algorithm sensitivity which increased from 56.5% in Period 1 to 86.9% in Period 2 (p < 0.001). The proportion of non-diseased in the screening group was 95.3% in Period 1 and 93.0% in Period 2. Accordingly, there was a decrease in the algorithm specificity from Period 1 to Period 2. In the last period a higher cumulative incidence of CIN2 + was observed in the HPV group both for those referred to colposcopy and to screening, 51.9% and 1.3% versus 38.9% and 0.9% for the Cytology group (Table II). A higher algorithm sensitivity was found in the HPV group than in the Cytology group, 93.6%, 72.9% (p < 0.001).

Table II. Follow-up on CIN 2 + for 3 years after triage for women returned to routine screening and referred to colposcopy.

Initial cytology	Group	Screening group		Referral group		Percentage of triaged women in referral group	Estimated percentage of all diseased* in referral group	Estimated percentage of all non-diseased^† in screening group
		Observed events during follow-up	Cumulative incidence (%) (95% confidence interval)	Observed events during follow-up	Cumulative incidence (%) (95% confidence interval)
All	Period 1^‡	1029	2.7 (2.5 − 2.9)	1484	42.0 (40.3 − 43.7)	7.7	56.5	95.3
	Period 2^§	337	1.0 (0.9 − 1.2)	2426	48.0 (46.6 − 49.4)	12.5	86.9	93.0
	HPV group^‖	114	1.3 (1.1 − 1.5)	1803	51.9 (50.2 − 53.6)	26.5	93.6	85.0
	Cytology group^¶	223	0.9 (0.8 − 1.1)	623	38.9 (36.5 − 41.5)	6.1	72.9	96.1
Unsatis factory	Period 1	225	0.9 (0.8 − 1.1)	135	13.7 (11.6 − 16.1)	3.4	34.2	97.0
	Period 2	61	0.4 (0.3 − 0.5)	140	21.5 (18.5 − 24.9)	3.3	66.6	97.4
	HPV group	6	0.3 (0.1 − 0.6)	75	39.4 (32.8 − 46.8)	7.9	92.3	95.1
	Cytology group	55	0.4 (0.3 − 0.5)	65	13.8 (10.9 − 17.2)	2.7	50.2	97.6
ASC-US	Period 1	551	5.6 (5.2 − 6.1)	820	53.9 (51.2 − 56.6)	12.0	56.8	93.7
	Period 2	169	1.5 (1.3 − 1.7)	1260	51.6 (49.6 − 53.7)	16.2	87.2	91.4
	HPV group	61	1.2 (0.9 − 1.5)	950	52.9 (50.5 − 55.3)	24.1	93.6	86.9
	Cytology group	108	1.7 (1.4 − 2.1)	310	47.4 (43.4 − 51.6)	8.9	73.1	95.0
LSIL	Period 1	253	8.7 (7.7 − 9.8)	529	51.7 (48.3 − 54.8)	23.9	65.2	85.8
	Period 2	107	3.1 (2.6 − 3.8)	1026	51.9 (49.6 − 54.2)	34.5	89.7	79.2
	HPV group	47	3.0 (2.3 − 4.1)	778	52.2 (49.6 − 54.8)	46.5	93.7	70.0
	Cytology group	60	3.3 (2.5 − 4.2)	248	51.3 (46.6 − 56.3)	20.5	80.3	88.5

*CIN2 + within 3 years; ^†No CIN2 + within 3 years; ^‡Initial cytology 1 January 1999 to 31 December 2001; ^§Initial cytology 1 July 2004 to 30 June 2008; ^‖HPV test for triage (Period 2); ^¶No HPV test for triage (Period 2).

For the screening groups the cumulative incidence of CIN2 + in Period 1 was 5.6% and 8.7% for ASC-US and LSIL screening cytology, respectively (Table II). In Period 2 the cumulative incidence was 1.5% and 3.1% for these groups (p < 0.001 and p < 0.001). For all categories of initial cytology the higher algorithm sensitivity was found in the HPV group. Follow-up for the estimates in Table II started at the date of referral to colposcopy or return to routine screening. For all women eligible for triage, follow-up from date of the initial cytology in Period 2 showed that the cumulative incidence of CIN2 + at three years was 1.0% [95% confidence interval (CI) 0.9–1.2%] for the group with initial unsatisfactory cytology, 9.8% (95% CI 9.4–10.2%) for the ASC-US group and 19.6% (95% CI 18.8–20.5%) for the LSIL group.

The CIN2 + rate was 1.85·10⁻² (95% CI 1.80–1.91·10⁻²) in Period 1 and 1.96·10⁻² (95% CI 1.91–2.02·10⁻²) in Period 2. The mean numbers of person years were 3.48 and 3.42, respectively.

In the HPV group the positivity rates of the HPV tests by the results of the initial cytology were as follows: 14.0% (378/2693) (unsatisfactory), 30.0% (2603/8674) (ASC-US) and 52.5% (1976/3766) (LSIL) (Table III). For all three groups there was a decreasing trend in positivity rate by age.

Table III. hrHPV positive (%) by age group and initial cytology.

Age (years)	Initial cytology						Total
	Unsatisfactory		ASC-US		LSIL
	Number of women	hrHPV-positive (%)	Number of women	hrHPV-positive (%)	Number of women	hrHPV-positive (%)	Number of women	hrHPV-positive (%)
25–34	896	193 (21.5)	2487	1211 (48.7)	1687	1031 (61.1)	5070	2435 (48.0)
35–44	913	112 (12.3)	2718	821 (30.2)	1225	614 (50.1)	4856	1547 (31.9)
45–54	591	54 (9.1)	2189	380 (17.4)	643	258 (40.1)	3423	692 (20.2)
55–69	293	19 (6.5)	1280	191 (14.9)	211	73 (34.6)	1784	283 (15.9)
Total	2693	378 (14.0)	8674	2603 (30.0)	3766	1976 (52.5)	15133	4957 (32.8)

Discussion

A successful triage algorithm is indicated by a high rate of CIN2 + among women in the referral group (PPV), i.e. those referred to colposcopy, and a very low rate in the screening group, i.e. the women recalled to ordinary screening as a result of triage. The introduction of hrHPV tests in triage in the Norwegian screening programme resulted in better separation of the women with respect to the risk of CIN2+. The new algorithm with HPV testing and cytology had higher sensitivity than the former algorithm based on cytology only. However, the new algorithm resulted in a modest decrease in specificity. It also resulted in a higher percentage of women referred to colposcopy, and thus to a higher workload.

These conclusions are suggested by the observed changes from Period 1, when repeat cytology was the only available triage, to Period 2 where cotesting for hrHPV was an option. The results from the comparison of the HPV group and the Cytology group in Period 2 corroborate that use of HPV testing in triage has caused at least part of the observed changes.

The decrease in three-year cumulative incidence of CIN2 + from Period 1 to Period 2 in the screening group contributed substantially to the better risk stratification in triage in the second period. This meant that risk of disease in women triaged back to routine screening were at more acceptable levels in Period 2. In the first period the risks were above 2.0% which has been suggested as an upper limit [22].

Comparison of results in Period 1 and 2 is the main part of the analysis. Since not all triage in the second period was performed with HPV cotesting, we computed linear extrapolations to see if these could be used as an estimate for the situation with complete adherence. Results for the separate HPV group and Cytology group in Period 2 were used for supplementary analysis.

We have to be aware of possible sources of bias in the comparison of Period 1 and 2, and between the HPV group and the Cytology group. Cytological results from Period 1 (1999–2001) were recoded in accordance with the Bethesda classification adapted from 2005 in cervical cancer screening in Norway. However, differences remain in the definition of unsatisfactory, and this definition had a broader content in the first period. Relatively more women had unsatisfactory as initial cytology in Period 1. We also observed that the ratio of LSIL cytology to ASC-US cytology is higher in Period 2 than in Period 1. Since the risk of precursors of cervical cancer is highly different in the three initial groups, these observations indicate a higher average risk in Period 2. Several of the measures we present are dependent on the risk level and comparison between the periods should be undertaken with analysis stratified on categories of initial cytology.

For some of the women triage was not attempted at all. As we classified women with no HPV test in Period 2 as belonging to the Cytology group, the number of incomplete screening was high in this group. Even when we restricted our analysis to women with complete triage, it is possible that systematic differences between the groups could have influenced our results. For each group of initial cytology the percentage of women triaged to the referral group were smaller in the cytology group in Period 2 than in Period 1. Within Period 2 there may have been systematic differences between those who underwent HPV testing and those who underwent cytology only. Specifically, it is possible that women with higher risk more often were selected for HPV triage in Period 2. Such a selection might have influenced the observed differences for measures that are dependent on the risk level (Table II).

Follow-up of later disease was performed by linkage of national registries of screening and disease and no adjustment for the higher diagnostic activity in the referral group was attempted.

Since unsatisfactory is seldom used as indication for triage in other programmes and international guidelines are given separately for ASC-US and LSIL, results for each initial cytology are discussed separately below.

Triage of initial LSIL cytology

From Period 1 to Period 2 the referral rate increased from 23.9% to 34.5%. Extrapolation of these results yields 45.1% with complete adherence since HPV cotesting was used for about half the women with complete triage. This is below the observed result 46.5% in the HPV group which we find reasonable since it was indicated that this was a selective group with risk of CIN2 + above the average. Cumulative incidence in the screening group decreased from 8.7% to 3.1%. Extrapolation of this result implies a decrease of 8.7–3.1 = 5.6% points from 3.1%, a negative, impossible value. It is likely that some change from Period 1 to Period 2 is not caused by introduction of HPV tests in triage. Cumulative incidence in the screening group is a parameter increasing with the risk level and we propose the observed value 3.0% in the HPV group as an upper limit for this parameter if complete adherence to the recommendations.

Current European guidelines for cervical cancer screening do not recommend HPV testing in triage for all women with LSIL as initial cytology result, because most women in the group are HPV test positive and the test will offer low discriminatory power [3]. This may, however, depend on whether HPV testing is conducted at the same time as the initial cytology (reflex HPV testing) or delayed, as in our study. Liquid-based cytology was very rare in Norway in 2005. Therefore, the national guidelines recommend delayed triage with a new cytology and hrHPV test six months after the initial cytology, also for the LSIL group.

The hrHPV positivity rate for the LSIL group in Norway was 52.5% (Table III). This is lower than observed in most international studies with reflex testing of HPV [14,23], and as explained above, is possibly due to the delayed HPV testing. We observed favourable changes in cumulative incidence of CIN2 + for the screening group and improved sensitivity from Period 1 to Period 2. The difference in these measures between the HPV group and Cytology group (Table II) supports that introduction of HPV in triage at least in part caused these changes. If the new guidelines with HPV testing were applied for all eligible women, we estimate that less than 50% of the LSIL group would be referred to colposcopy. Higher referral rates are indicated with reflex HPV testing [14,24].

For the LSIL group the three-year risk of CIN2 + was approximately 20%. A risk for which referral to colposcopy should be considered [22]. If one chooses to triage this group, high sensitivity is paramount and it may be defendable to refer more than 40% to colposcopy for achieving optimal sensitivity. In our case, delayed triage of the LSIL group was not optimal; there was a 3% risk of CIN2 + among those who were sent back to screening. Some will argue that a group with such a risk needs prolonged surveillance [22] so our results illustrate the well-known problems of finding a perfect management for the LSIL group in all respects.

Triage of initial ASC-US cytology

From Period 1 to Period 2 the referral rate increased from 12.0% to 16.2%. Extrapolation of these results yields 20.4% with complete adherence since HPV cotesting was used for about half the women with complete triage. This is below the observed result 24.1% in the HPV group which probably is a result influenced by selective recruitment to the group. The cumulative incidence of CIN2 + in the screening group decreased from 5.6% to 1.5% from Period 1 to Period 2 and extrapolation results in negative, impossible values as for women with LSIL as initial cytology. It is likely that some of the improvement from the first to the second period is not caused by the introduction of HPV test in triage. In analogy with the result for the LSIL group we think that the observed value of 1.2% in the HPV group may serve as an upper limit for possible values if complete adherence.

HPV triage is recommended for women with an initial ASC-US cytology [3]. The positivity rate of HPV tests in NCCSP with delayed triage was about 30%. This is below levels reported by others [14,23]. Reflex HPV testing would be expected to result in a higher percentage of HPV-positive women.

Use of hrHPV testing in triage of the ASC-US group may result in more than 20% of women being referred to colposcopy. However, this triage with delayed HPV testing gave a satisfying separation in groups with respect to later disease. At least 75% were returned to routine screening without further examinations, and the three-year cumulative incidence of CIN2 + for this group was estimated to be as low as 1.2%. Further, the incidence of CIN2 + in the referral group (PPV) was above 50%. Thus, the low risk in the screening group and the high risk in the referral group meant that a satisfactory separation was achieved with the new algorithm. The hrHPV positivity rate for the younger age group 25–34 years was 48.7%, but even with this high positivity rate, one could argue that triage with delayed HPV test seems tenable.

Triage of initial unsatisfactory cytology

Unsatisfactory cytology has been used as an indication for triage in Norway both before and the first four years after the introduction of HPV tests in triage. A recommendation for such treatment of those women cannot be found in the European guidelines [3]. Former analysis of Norwegian data has demonstrated an increased risk of CIN2 + for this group of screened women [25]. Our results suggest that adjunct HPV testing was used in less than 10% of cases with unsatisfactory screening cytology. These results provide no good clues to predictions of trends in the situation with complete adherence to the recommendations.

In our analysis triage with cotesting had more favourable predictive values than triage with repeated smears only (Table II). The sensitivity of HPV triage was equally high for this group and for women with ASC-US and LSIL as initial cytology. There are, however, some doubts if this group is at elevated risk. The three-year risk of CIN2 + in Period 2 was 1% for the unsatisfactory group, a risk some authors find acceptable for advising the women to return to routine screening [22,23].These results support the Norwegian guideline from 2009 for repeat cytology alone after unsatisfactory cytology. The women are just recommended to have a new cytology within six months.

Adherence to recommendations

After HPV tests were recommended for triage in 2005, not all triage were performed in accordance with the new guidelines. Some medical doctors simply continued the use of repeat cytology. Among those with ASC-US or LSIL at initial cytology, HPV tests were used in about 60% of the cases at the end of the study period. Given the promising results with HPV triage, its use should be encouraged among doctors and laboratories.

A challenge with the delayed triage was that it likely reduced adherence to recommendations, i.e. some women did not make an appointment for the subsequent smear/HPV test. The analysis revealed that for more than 30% of the triage episodes the surveillance with new tests was not executed properly. This may reduce the benefits of triaging and lower the effectiveness of the screening programme. Better information to the women and their medical doctors might improve adherence to the recommended guidelines. The result, however, necessitates a discussion of pros and cons of delayed triage testing versus reflex testing in the programme.

Conclusion

Introduction of HPV testing in triage in the NCCSP in 2005 has resulted in a better risk stratification of women. The increase in referral to colposcopy seems acceptable. It also appears as if additional benefit could be gained if all physicians adhered to the current guidelines.

Acknowledgements

We thank Bente Kristin Johansen, previous leader of the NCCSP, for participating in planning of this study, Jan Nygård, CRN for contributing to the data preparation for the study, and Bjarte Aagnes, CRN for assistance with the data analysis.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.