Validity of asthma diagnoses in the Danish National Registry of Patients, including an assessment of impact of misclassification on risk estimates in an actual dataset

Figures & data

Table 1 General format for contingency tables

	Misclassified values			Corrected values
	Asthma			Asthma
	Yes	No	Total	Yes	No	Total
Skin cancer cases	A1*	A0*	A	A1	A0	A
Person-years (PY)	PY1*	PY0*	PY	PY1	PY0	PY
	$\text{IRR}*=\frac{\text{A}1*/\text{PY}1*}{\text{A}0*/\text{PY}0*}$			$\text{IRR}=\frac{\text{A}1/\text{PY}1}{\text{A}0/\text{PY}0}$

Table 2 Validity of the diagnosis codes of asthma in the DNPR, January 1st, 1977 to December 31st, 2003

	Military Conscription Registry
Danish National Registry of Patients	With asthma diagnosis	Without asthma diagnosis
With asthma diagnosis	604	321
Without asthma diagnosis	754	20,498
Total	1,358	20,819
Sensitivity (95% CI):	604/1,358 = 0.44 (0.42–0.47)
Specificity (95% CI):	20,498/20,819 = 0.98 (0.98–0.99)

Abbreviations: CI, confidence intervals; DNRP, Danish National Registry of Patients.

Table 3 Positive predictive value of the diagnosis codes of asthma in the DNRP, January 1st, 1977 to December 31st, 2003 by hospital contact type

	Danish National Registry of Patients
	Military conscription registry	With asthma diagnosis	Without asthma diagnosis
Asthma total	With asthma diagnosis	604	754
	Without asthma diagnosis	321	20,498
	Total	925	21,252
	PPV (95% CI):		604/925 = 0.65 (0.62–0.68)
Asthma hospitalization (ie, inpatient stay)	With asthma diagnosis	406	754
	Without asthma diagnosis	215	20,498
	Total	621	21,252
	PPV (95% CI)		406/621 = 0.65 (0.62–0.69)
Asthma outpatient clinic contact	With asthma diagnosis	187	754
	Without asthma diagnosis	102	20,498
	Total	289	21,252
	PPV (95% CI)		187/289 = 0.65 (0.59–0.70)
Asthma emergency department visit	With asthma diagnosis	11	754
	Without asthma diagnosis	4	20,498
	Total	15	21,252
	PPV (95% CI)		11/15 = 0.73 (0.51–0.96)

Abbreviations: CI, confidence interval; DNRP, Danish National Registry of Patients; PPV, positive predictive value.

Table 4 A–C The impact of misclassification on various risk estimates if the DNPR were used to identify asthma patients’ risk of skin cancers, misclassified and corrected for misclassification

	Misclassified values			Corrected values
	Asthma			Asthma
	Yes	No	Total	Yes	No	Total
A. Malignant melanoma
Cases with asthma	6.00	159.00	165	7.85	157.15	165
Person-years at risk	48,744.96	1,120,199.04	1,168,944	69,847.59	1,099,096.41	1,168,944
	IRR* = 0.8679			IRR = 0.7862
B. Basal cell carcinoma
Cases with asthma	40.41	821.59	862	61.74	800.26	862
Person-years at risk	48,744.96	1,120,199.04	1,168,944	69,847.59	1,099,096.41	1,168,944
	IRR* = 1.1302			IRR = 1.2140
C. Squamous cell carcinoma
Cases with asthma	7.34	115.66	123	12.42		123
Person-years at risk	48,744.96	1,120,199.04	1,168,944	69,847.59	1,099,096.41	1,168,944
	IRR* = 1.4582			IRR = 1.7669

Abbreviation: IRR, incidence rate ratio.

Figure 1A–C The impact of misclassification on various risk estimates if the Danish National Patient Registry were used to identify asthma patients’ risk of skin cancers for varying values of sensitivity and specificity.

Abbreviation: IRR, incidence rate ratios.