Validity of diagnostic codes and prevalence of psoriasis and psoriatic arthritis in a managed care population, 1996-2009

Abstract

Background: Few population-based studies have reported the prevalence of psoriatic disease.

Objective: We validated computerized diagnoses to estimate the prevalence of psoriasis and psoriatic arthritis.

Method: We identified adults with ≥1 ICD-9 diagnosis codes of 696.0 (psoriatic arthritis) or 696.1 (psoriasis) in clinical encounter data during 1996-2009 and used chart review to confirm the diagnoses in random samples of patients. We then used the best performing case-finding algorithms to estimate the point prevalence of psoriasis and psoriatic arthritis.

Results: The number of persons with a diagnosis for psoriasis (ICD-9 code 696.1) was 87 827. Chart review of a random sample of 101 cases with at least one dermatologist-rendered psoriasis code revealed a positive predictive value (PPV) of 90% (95% CI, 83-95) with sensitivity of 88% (95% CI, 80-93). Psoriatic arthritis (code 696.0) was recorded for 5187 patients, with the best performing algorithm requiring ≥2 diagnoses recorded by a rheumatologist or ≥1 diagnosis recorded by a rheumatologist together with ≥1 psoriasis diagnoses recorded by a dermatologist; the PPV was 80% (95% CI, 70-88) with sensitivity 73% (95% CI, 63-82). Among KPNC adults, the point prevalence of psoriasis, with or without psoriatic arthritis, was 939 (95% CI, 765-1142) per 100 000, and the overall prevalence of psoriatic arthritis, with or without psoriasis, was 68 (95% CI, 54-84) per 100 000.

Conclusion: Within an integrated health care delivery system, the use of computerized diagnoses rendered by relevant disease specialists is a valid method for identifying individuals with psoriatic disease.

Keywords: computerized medical information; epidemiology; incidence, prevalence, health maintenance organizations; pharmacoepidemiology; psoriasis; psoriatic arthritis.

Figure 1. Age- and sex-specific point prevalence¹ per 100,000 persons (shown on Y-axis) of psoriasis with or without psoriatic arthritis (N=55,770) and psoriatic arthritis with or without psoriasis (N=2,941). Men and women aged ≥18 years. Kaiser Permanente Northern California, December 31, 2009

¹The prevalence proportions are corrected for the sensitivity and positive predictive value of the validated case-finding algorithms. PsO = psoriasis with or without psoriatic arthritis. PsA = psoriasis with or without psoriasis.