Cryptococcal antigen titers and semi-quantitative assay scores among people with HIV-associated cryptococcal antigenemia

TL;DR

This study shows that most HIV patients with cryptococcal antigenemia in South Africa have high antigen levels, indicating a high risk of meningitis, but few undergo the recommended spinal tap to confirm it.

Contribution

The study introduces the use of single-strip semi-quantitative CrAg testing as a rapid, risk-stratification tool for cryptococcal meningitis in resource-limited settings.

Findings

63% of patients had a cryptococcal antigen titer of ≥160, indicating high risk of meningitis.

Only 31% of patients had a lumbar puncture within 28 days of a positive test, and 60% of those had confirmed meningitis.

Semi-quantitative CrAg testing can identify high-risk patients quickly, reducing the need for full titer testing.

Abstract

While South African guidelines recommend a lumbar puncture (LP) to exclude cryptococcal meningitis (CM) among all people with a newly positive cryptococcal antigen (CrAg) test, irrespective of CM symptoms, this is not always feasible. High blood CrAg lateral flow assay (LFA) titers are associated with concurrent CM and increased mortality. Single-strip CrAg semi-quantitative (SQ) tests could risk-stratify people with antigenemia. Consecutive fresh LFA-positive remnant plasma samples from a CD4 laboratory network collected between April and July 2021 were retested. We described LFA titers, CrAgSQ scores, and the proportion with cerebrospinal fluid (CSF) collected 28 days before or after a positive CrAg screening test. Of 2,240 re-tested plasma samples from unique patients, 2,166 (97%) were confirmed LFA-positive. The median LFA titer was 640 (IQR, 40–5,120), 63% (1,354/2,166) had a titer of ≥160, and 52% (1,124/2,166) had SQ scores of ≥3+. Only 31% (662/2,166) had a CSF sample collected 28 days before or after a CrAg LFA-positive test; 60% (398/662) had confirmed CM. More than half of the people with cryptococcal antigenemia had a blood CrAg titer of ≥160 or a CrAgSQ score of ≥3+, both previously shown to confer a high risk of concurrent CM. Of 3 in 10 who had an LP, most had CM, suggesting that meningitis symptoms prompted LP. Healthcare worker support/training is required to improve adherence to the universal LP recommendation. When immediate LP is not feasible, blood CrAgSQ testing can rapidly identify people at the highest risk of CM who require urgent referral for LP.

A majority of patients with HIV-associated cryptococcal antigenemia identified through a large screening program in South Africa had high cryptococcal antigen titers and thus an elevated risk of concurrent meningitis and death. Despite this, a relatively small proportion had a lumbar puncture to definitively exclude meningitis. Routine CrAg semi-quantification can help to stratify patients at higher risk for meningitis and guide clinicians’ management, but performing a full range of titers for all CrAg-positive blood samples increases costs and is labor-intensive. An alternative approach is to use a single test strip, which yields a semi-quantitative score.

Figures