Diagnostic Precision of QuantiFERON-TB Gold Plus Immunoassay Benchmarked Against Mycobacterial Cultivation and Nucleic Acid Amplification: A Retrospective Evaluation in 187 Patients

Mycobacterium tuberculosis infection continues to impose disproportionate global health burdens, demanding precise and expeditious diagnostic platforms. Fourth-generation interferon-gamma release assays (IGRAs), exemplified by QuantiFERON-TB Gold Plus (QFT-Plus), offer immunological detection capabilities, whereas mycobacterial cultivation and nucleic acid amplification technologies constitute reference benchmarks for active tuberculosis (TB) confirmation. Critically, IGRAs such as QFT-Plus detect T-cell immune sensitization to M. tuberculosis antigens—a response present across the spectrum from latent to active infection—and cannot independently confirm or exclude active TB disease; their results require integration with clinical, radiological, and microbiological evaluation. Our investigation assessed QFT-Plus diagnostic precision against microbiological gold standards within routine clinical practice. Methods: Retrospective analysis encompassed 187 unique patients undergoing concurrent QFT-Plus immunoassay and either mycobacterial culture (MGIT 960 plus Löwenstein–Jensen media) or GeneXpert MTB/RIF molecular testing during December 2024 through December 2025 at MDLab Central Laboratory, Riyadh, Saudi Arabia. Performance metrics—including true-positive fraction, true-negative fraction, predictive values, likelihood ratios, and Cohen’s kappa—were computed using Wilson score 95% confidence intervals. Results: Within 187 unique patients (mean age 49.7 ± 21.9 years; 53.5% male; 68.4% Saudi nationals), QFT-Plus yielded a detection sensitivity of 88.24% (95% CI: 65.66–96.71%), exclusion specificity of 80.00% (95% CI: 73.35–85.32%), positive predictive value of 30.61%, and negative predictive value of 98.55%. Overall concordance reached 80.75% with fair-to-moderate agreement (Cohen’s κ = 0.3694, p < 0.001). Receiver operating characteristic area under the curve measured 0.868. Seventeen patients (9.09%) achieved microbiological TB confirmation. Age demonstrated significant influence on QFT-Plus outcomes (p = 0.009), with TB-positive cases averaging 46.2 years versus 50.1 years in TB-negative cohorts. A strong inverse correlation was observed between GeneXpert cycle threshold values and culture time-to-positivity (Spearman’s ρ = 0.78, p < 0.001), reflecting greater mycobacterial burden in GeneXpert-positive specimens. QFT-Plus demonstrates high sensitivity (88.24%) and strong negative predictive value (98.55%) for microbiologically confirmed tuberculosis, supporting its role as an adjunctive immunological test in TB evaluation workflows. As QFT-Plus detects T-cell immune sensitization to M. tuberculosis antigens—a response present in both latent and active infection—it cannot independently diagnose or exclude active TB disease. The elevated rate of QFT-positive/microbiologically negative results (34/187; 18.2%) most plausibly reflects latent TB infection rather than assay failure. All QFT-Plus results must be interpreted within the clinical context, integrating radiological and microbiological findings; positive results require confirmatory microbiological testing before treatment decisions are made. Age-stratified interpretation with complementary culture or molecular testing optimizes overall diagnostic precision. This discordant rate of 18.2% aligns with published LTBI prevalence estimates of 15–25% in similar low-to-intermediate TB-burden clinical populations, and is consistent with WHO LTBI prevalence data for the Gulf Cooperation Council region.