Evaluation of the diagnostic performance of a semi-quantitative lateral flow assay for the detection of <i>Cryptococcus</i> antigen in serum and cerebrospinal fluid specimens

Cryptococcosis predominantly affects immunocompromised individuals, particularly those with advanced HIV disease, with meningitis being the most severe form and linked to high mortality. Diagnosis typically relies on rapid Cryptococcus antigen (CrAg) testing, and antigen titer quantification helps in early detection and assessing disease severity. However, conventional titer methods are often more expensive than qualitative antigen detection. This study assessed the diagnostic performance of a semi-quantitative lateral flow assay (CrAgSQ LFA, IMMY™) for CrAg detection in serum and cerebrospinal fluid (CSF) collected between 2014 and 2022. The CrAgSQ LFA was compared to the standard CrAg LFA (IMMY™) and Clarus Cryptococcal Ag enzyme immunoassay (EIA-CrAg, IMMY™). The CrAgSQ LFA demonstrated 100% sensitivity and specificity in both serum and CSF, with perfect agreement (kappa 1.00) with the CrAg LFA. When comparing the CrAgSQ LFA with the titer measurement results obtained using CrAg LFA, titers ranged as follows: in category 1+, from 1:2 to 1:20; in 2+, from 1:5 to 1:40; in 3+, from 1:20 to 1:2560; and in 4+, from 1:320 to 1:2560. Titer results for the CrAgSQ LFA aligned well with CrAg LFA, and operator agreement was strong, with weighted kappa values of 0.926 and 0.966. The CrAg-EIA showed a sensitivity of 84% and specificity of 100% using the manufacture cutoff (>0.265), which improved to 96% sensitivity with an optimized cutoff value (>0.145). Overall, the CrAgSQ LFA demonstrated high accuracy and reliability, suggesting it could be a valuable tool for diagnosing cryptococcosis in the Americas.