Utility of Cerebrospinal Fluid Unstimulated Interferon-Gamma (IRISA-TB) as a Same-Day Test for Tuberculous Meningitis in a Tuberculosis-Endemic, Resource-Poor Setting

Abstract

Background: Tuberculous meningitis (TBM) mortality is high and current diagnostics perform suboptimally. We evaluated the diagnostic performance of a DNA-based assay (GeneXpert Ultra) against a new same-day immunodiagnostic assay that detects unstimulated interferon-gamma (IRISA-TB).

Methods: In a stage 1 evaluation, IRISA-TB was evaluated in biobanked samples from Zambia (n = 82; tuberculosis [TB] and non-TBM), and specificity in a South African biobank (n = 291; non-TBM only). Given encouraging results, a stage 2 evaluation was performed in suspected TBM patients from Zimbabwe and Malawi (n = 668). Patients were classified as having definite, probable or possible TBM, or non-TBM based on their microbiological results, cerebrospinal fluid (CSF) chemistry, and whether they received treatment.

Results: In the stage 1 evaluation, sensitivity and specificity of IRISA-TB were 75% and 87% in the Zambian samples, and specificity was 100% in the South African samples. In the stage 2 validation, IRISA-TB sensitivity (95% confidence interval [CI]) was significantly higher than Xpert Ultra (76.2% [55.0%-89.4%] vs 25% [8.9%-53.3%]; P = .0048) when trace readouts were considered negative. Specificity (95% CI) was similar for both assays (91.4% [88.8%-93.4%] vs 86.9% [83.4%-89.8%]). When the Xpert Ultra polymerase chain reaction product was verified by sequencing, the positive predictive value of trace readouts in CSF was 27.8%. Sensitivity of IRISA-TB was higher in human immunodeficiency virus (HIV)-infected versus uninfected participants (85.8% vs 66.7%).

Conclusions: As a same-day rule-in test, IRISA-TB had significantly better sensitivity than Xpert Ultra in a TB/HIV-endemic setting. An immunodiagnostic approach to TBM is promising, and further studies are warranted.

Keywords: GeneXpert Ultra; accuracy; diagnosis; interferon gamma; tuberculous meningitis.

Figure 1.

Study overview outlining stage 1 evaluation (n = 253) and stage 2 evaluation (n = 686). Abbreviations: CSF, cerebrospinal fluid; IFN-γ, interferon gamma; IRISA-TB, inter-gam ultrasensitive rapid immuno-suspension assay; TB, tuberculosis; TBM, tuberculous meningitis.

Figure 2.

Performance data for stage 1 evaluation to assess specificity in biobanked South African samples. A, Receiver operating characteristic curve of IRISA-TB performance analyzed by human immunodeficiency virus (HIV) status. B, Performance data of IRISA-TB analyzed by HIV status. C, Specificity of IRISA-TB was assessed in those with clinical presentation of meningism or neurological symptoms (n = 171). An embedded study evaluated the specificity of IRISA-TB in those whose cerebrospinal fluid was normal (n = 120). D, Scatter plot depicting the performance of IRISA-TB in ruling out tuberculous meningitis (n = 171), analyzed by HIV status and age. Cutoff of 13 pg/mL is depicted by a dashed black line. Abbreviations: AUC, area under the curve; CI, confidence interval; DOR, diagnostic odds ratio; HIV, human immunodeficiency virus; IRISA-TB, inter-gam ultrasensitive rapid immuno-suspension assay; NLR, negative likelihood ratio; NPV, negative predictive value; PLR, positive likelihood ratio; PPV, positive predictive value; PWH, people with human immunodeficiency virus.

Figure 3.

IRISA-TB performance (definite and non-tuberculosis) with trace readouts included (considered positive). A, Performance of IRISA-TB and Xpert Ultra in stage 2 evaluation, analyzed by human immunodeficiency virus (HIV) status (P < .05). B, Receiver operating characteristic curve of IRISA-TB performance in the stage 2 cohort analyzed by HIV status. Trace results were considered a positive Xpert Ultra in both A and B. Abbreviations: AUC, area under the curve; CI, confidence interval; DOR, diagnostic odds ratio; HIV, human immunodeficiency virus; IRISA-TB, inter-gam ultrasensitive rapid immuno-suspension assay; N/A, not applicable; NLR, negative likelihood ratio; NPV, negative predictive value; PLR, positive likelihood ratio; PPV, positive predictive value; PWH, people with human immunodeficiency virus.

Figure 4.

Clarification of Xpert Ultra Trace readouts using Sanger Sequencing. A, The positive predictive value of the Xpert Trace result in cerebrospinal fluid, using available retained cartridges, was 27.8%. B, As a control and to validate the value of the sequencing approach, sputum samples subjected to Xpert Ultra were also validated by Sanger sequencing. The predictive value of Xpert Ultra was 100%. Abbreviations: Mtb, Mycobacterium tuberculosis; TBM, tuberculous meningitis.