The reliability of tuberculin skin test in the diagnosis of latent tuberculosis infection in psoriasis patients: A case‐control study

BACKGROUND: Tuberculin skin test (TST), which is used in the diagnosis of latent tuberculosis infection may cause Koebner's phenomenon and false positive results in psoriasis patients. OBJECTIVE: The purpose of this study is to compare TST with QuantiFERON-TB Gold Plus (QFT-plus) test in psoriasis patients, and to determine the effects of psoriasis on TST results. METHODS: Ninety-two psoriasis patients and 30 control subjects were included in the study. QFT-plus test, TST and prick test to distinguish the increase of induration because of the skin trauma were performed on both groups. The demographics, risk factors for latent tuberculosis infection, BCG vaccination history, Koebner's history, psoriasis severity and treatment history of the patients were recorded. The effects of these variables on test results were investigated by comparing those with control group. The criteria of National Tuberculosis Diagnosis and Treatment Guideline were used in the evaluation of test results, and threshold value of positivity for TST was taken as 10 mm in BCG-vaccinated patients who are planned to start biological treatment. RESULTS: Prick test results were negative in the control group. There was no significant relation between the results of prick test and TST induration diameters in the patient group. Although TST positivity was significantly higher in patients (62%) compared to control group (33%), QFT-plus test results were not statistically different between two groups. Agreement between two tests was determined to be low in patient group with 48% (K=0.1), and it was determined to be moderate with 77% in control group (K=0.4). QFT-plus test was found to be negative in 46 of 57 TST-positive patients (80.7%) in patient group. It was determined in both groups that vaccination did not have any effect on test results. When threshold value was lowered to 5 mm in patient group without considering BCG reaction, the number of TST-positive patients increased to 65 from 57. Mean TST induration diameter was 10 mm and 14 in cases with mild and moderate-severe clinical manifestation, respectively (p=0.04). However, no effect of disease period and treatment was determined on both test results. CONCLUSION: TST positivity was higher in psoriasis patients compared to control group. It was considered due to the increased reaction of the skin to mycobacterial antigens rather than the Koebner's response. While TST results were not affected by BCG, it was concluded that a 10 mm threshold value of positivity was a suitable approach in order to reduce the number of patients receiving unnecessary preventive treatment in patients who are considered to initiate biologic agents. Furthermore, it was also concluded that QFT-plus test may be preferred in psoriasis patients since it is applied in vitro, its specificity is higher and not affected by disease severity. This article is protected by copyright. All rights reserved.