Reasons for ordering laboratory tests and relationship with frequency of abnormal results

Figures & data

Table I. Reference values, action limits, and abnormal results.

Test	N tests	% abnormal test results according to reference values (95% CI)	Action limits	% markedly abnormal test results according to action limits (95% CI)
ALAT (U/1)	440	10% (8%–13%)	<70	3% (l%–4%)
Cholesterol (mmol/1)	584	38% (34%–41%)	<8.0	3% (l%–4%)
Creatinine clearance (ml/min/1.73 m^2)^1	780	61% (58%–65%)	<60	21% (18%–24%)
CRP (mg/1)	175	38% (31%–45%)	<50	5% (l%–8%)
ESR (mm/hr)	877	22% (19%–25%)	<40	5% (3%–6%)
Ferritin (μg/l)	57	47% (34%–60%)	15–1000	35% (23%–47%)
Gamma GT (U/1)	239	24% (19%–30%)	<90	6% (3%–9%)
Glucose (mmol/1)	949	8% (6%–9%)	3.0–11.0	2% (l%–3%)
Hemoglobin (mmol/1)	878	13% (11%–16%)	♀6.5–10.5, ♂ 7.5–11.5	2% (l%–3%)
Leukocytes (*10e9/l)	574	13% (10%–25%)	2.5–15.0	2% (l%–3%)
MCV (fl)	716	6% (5%–8%)	75–105	1% (0%–2%)
Potassium (mmol/1)	334	16% (12%–20%)	3.0–5.5	1% (0%–3%)
Sodium (mmol/1)	137	7% (2%–ll%)	130–150	0%
TSH (mU/1)	660	11% (9%–13%)	0.1–8.0	5% (3%–7%)

Note: ¹MDRD formula, adjusted to the laboratory method for creatinine measurement (jaffe/enzymatic).

Table II. Distribution of abnormal test results at test level.¹

	N tests	% abnormal test results according to reference values	% markedly abnormal tests according to action limits
Patients (n= 1484)	7400	20.7%	4.9%
Sex
Men	2857	21.4%	4.3%
Women	4543	20.3%	5.3%
Age
< 40 years	1747	13.7%	2.5%
40–60 years	2773	19.8%	4.0%
> 60 years	2880	25.8%	7.4%
Reason for ordering tests
Confirm/determine treatment	725	27.2%	9.1%
Exclude/physician's uncertainty	3654	18.6%	4.2%
Reassure/patient's request	1139	13.4%	2.0%
Monitoring (screening/check-up)	1761	27.0%	6.6%
Other reasons	33	15.2%	3.0%
Estimated pretest probability^2
Definitely no disease	541	13.9%	2.2%
Probably no disease	1990	14.2%	2.6%
Possibly disease	1651	19.9%	4.5%
Probably disease	1127	24.3%	7.0%
Definitely disease	196	34.7%	14.3%
Presumed diagnosis^2
A97, P, Z	1083	13.3%	2.7%
Other ICPC codes	4350	20.1%	4.9%

Notes: ¹Limited to the 14 tests in Table I. ²The physicians did not estimate a pretest probability or mention a presumed diagnosis if they ordered the tests for screening or check-up.

Table III. Distribution of abnormal result at patient level.¹

Reasons for ordering tests	N patients	% patients with abnormal results according to reference values (95% CI)	% patients with markedly abnormal results according to action limits (95% CI)
Patients	1484	64% (61%–66%)	21% (19%–23%)
Sex
Men	591 (40%)	63% (60%–67%)	18% (15%–21%)*
Women	893 (60%)	64% (61%–67%)	23% (20%–6%)
Age
< 40 years	343 (23%)	48% (42%–53%)***	10% (7%–13%)***
40–60 years	582 (39%)	60% (57%–64%)	16% (13%–20%)
> 60 years	559 (38%)	77% (74%–81%)	33% (29%–37%)
Reason for ordering tests	16 missing
Confirm/determine treatment	167 (11%)	68% (61%–75%)***	30% (23%–37%)**
Exclude/physician's	651 (44%)	63% (59%–66%)	20% (17%–23%)
Uncertainty	207 (14%)	53% (46%–60%)	11% (7%–15%)
Reassure/patient's request	434 (29%)	69% (65%–73%)	24% (20%–27%)
Monitoring (screening/check-up)	9 (1%)	33% (7%–69%)	11% (0%–48%)
Other reasons
Estimated pretest probability^2	28 missing
Definitely no disease	97 (9%)	58% (48%–68%)***	12% (6%–19%)***
Probably no disease	353 (34%)	54% (48%–59%)	14% (10%–17%)
Possibly disease	301 (29%)	66% (60%–71%)	22% (17%–27%)
Probably disease	218(21%)	69% (63%–75%)	28% (22%–33%)
Definitely disease	53 (5%)	70% (57%–82%)	40% (26%–53%)
Presumed diagnosis^2	38 missing
A97, P, Z	173 (17%)	58% (51%–66%)	16% (10%–21%)
Other ICPC codes	839 (83%)	62% (59%–65%)	21% (18%–24%)

Notes: * p<0.05 ** p<0.01 *** p<0.001. ¹Limited to the 14 tests in Table I. ²The physicians did not estimate a pretest probability or mention a presumed diagnosis if they ordered the tests for screening or check-up.