The Use of Purified Protein Derivative (PPD) Response in Elispot <scp>IGRA</scp> in the Diagnosis of Nontuberculous Mycobacterial Cervical Lymphadenitis

The diagnosis of lymphadenitis caused by nontuberculous mycobacteria (NTM) can be difficult, time consuming, and may need invasive investigations. The aim of this study was to analyze the use of a simple blood test and PPD stimulation in the diagnosis of NTM infections in non-BCG-vaccinated children and to determine the cut-off value for PPD response. For this purpose, the result of a modified Elispot IGRA assay and clinical data from 939 pediatric patients tested in 2011-2015 at the Helsinki University Hospital were reviewed retrospectively. Altogether, 45 children with NTM cervical lymphadenitis were identified. Fifteen of the cases were confirmed by culture and comprised the study group for the PPD cut-off value calculation. One hundred forty-one healthy, non-BCG-vaccinated children with recent contact with an MTB positive case comprised the control group. The cut-off value for PPD response for the diagnosis of NTM cervical lymphadenitis was 29 spots/million lymphocytes. The PPD antigen in the Elispot IGRA test can also be used in the evaluation of the BCG vaccination status. The PPD cut-off for BCG vaccination was 36 spots/million lymphocytes. In conclusion, the PPD stimulation response is a useful minimally invasive diagnostic method in the diagnosis of NTM cervical lymphadenitis in non-BCG-vaccinated children.