Comparison of Interferon Gamma Release Assay and Tuberculin Skin Test for Diagnosis of Latent Tuberculosis in Psoriasis Patients Planned for Systemic Therapy.

BACKGROUND

Latent tuberculosis infection (LTBI) is a common yet difficult problem to diagnose in tuberculosis endemic countries. Both tuberculin skin test (TST) and interferon-gamma release assay (IGRA) are used for the diagnosis of LTBI.

AIMS

The aim of the study is to compare TST and IGRA in patients planned for systemic treatment of psoriasis.

METHODS

It was a diagnostic study conducted in a tertiary care centre during the study period from January 20 to December 20. Patients more than 18 years of age with chronic plaque psoriasis planned for systemic therapy were included. Psoriasis area severity index (PASI), history of tuberculosis in past or family and BCG vaccination were recorded. Complete blood count, radiograph of the chest, tuberculin skin test and interferon-gamma release assay were performed in all patients. Statistical analysis was performed using statistical package for social sciences (SPSS version 20, Chicago).

RESULTS

A total of 75 patients, including 48 males and 27 females, were included in the study. The mean age and mean duration of disease were 46.08 (±12.16) and 4.59 (±3.8) years, respectively. Seventy-one (94.6%) patients had BCG scar, and two (2.6%) had a history of tuberculosis in a family member. The TST and IGRA were positive (>10 mm) in 23 (30.6%) and 16 (21.3%) patients, respectively. Either TST or IGRA was positive in 28 (37.3%) patients. Out of these 28 patients, concordance was seen in 11 (39.2%) and discordance in 17 (60.7%). Discordance was TST+/IGRA - in 12 (42.8%) and TST-/IGRA + in five (17.8%) patients. Abnormality in radiograph of the chest and computed tomography (CT) scan of the chest were seen in five (6.6%) and nine (12%) patients, respectively. The patients with either TST or IGRA + were more likely to have abnormal chest radiographs than those who were TST-/IGRA- (OR: 11.3, 95% CI: 1.24-102.3, = 0.03). The TST and IGRA showed fair agreement ( = 0.364, = 0.003). ROC curve was plotted for the absolute value of TST in mm considering IGRA as the gold standard. The area under the curve was 0.805 (95%CI: 0.655-0.954). For the TST positivity cut-off of 10 and 15 mm, specificity was 77.3% and 95.5%, respectively; the sensitivity was 68.8% irrespective of the cut-off value.

LIMITATION

Small sample size and lack of follow-up are the biggest limitations of the study. The lack of a gold standard in the diagnosis of LTBI is an inherent yet unavoidable flaw in the study.

CONCLUSION

Reactivation of LTBI is a concern in a patient planned for immunosuppressive therapy. We suggest the use of both TST and IGRA rather than two-step testing (TST followed by IGRA) or IGRA alone for the diagnosis of LTBI, especially in patients with a high risk of reactivation. The positivity on either test should prompt further evaluation and treatment decisions should be taken considering the risk-benefit ratio of treatment rather than test results alone.