Diagnostic value of pathergy test in Behcet's disease according to the change of incidence over the time
Pathergy test (PT) is used for the diagnosis of Behcet's disease (BD). It is a criterion in many classification/diagnosis criteria. PT is mainly seen in BD but can be seen in other conditions too. PT has been reported with high frequency from most countries along the Silk Road. The sensitivity of pathergy phenomenon (PP) is declining over the time. The aim of this study was to look for the diagnostic value of PT in the past and at the present time. The BD registry (Rheumatology Research Center, Tehran University of Medical Sciences) has the data of 6,607 BD and 4,292 control patients. Patients and controls were divided in four groups of 1,650 BD and 1,073 controls. Sensitivity, specificity, positive and negative predictive value (PPV-NPV), positive and negative likelihood ratio (PLR-NLR), diagnostic odds ratio (DOR), and Youden's index (YI) were calculated for each group. The first and the fourth quartiles were compared. Sensitivity of PT decreased from 64.2% (first quartile) to 35.8% (fourth quartile). Specificity improved from 86.6% to 98.4%. PPV improved from 82.7% to 95.7%. NPV decreased from 82.7% to 60.5%. PLR improved from 4.8 to 22.4, while NLR deteriorated from 0.41 to 0.65. DOR improved from 11.6 to 34.3. Yuden's index worsened from 0.5 to 0.34. Although sensitivity of PP decreased, the increase of specificity is a plus value for diagnosis. As a result, PPV, PLR, and DOR improved, while NPR, NLR, and YI deteriorated. Although the pathergy test lost its sensitivity during the past 35 years, it has not lost its value as a diagnostic test, improving many of its characteristics. In a practical view, the chances of getting a positive test have decreased over the time. However, a positive test is rather the synonym of Behcet's disease, with a probability of 98.4%.