Whole Blood Volume-Based Absolute Quantification of HTLV-1 Proviral Load: A Comparative Method Evaluation Study

The proviral load of human T-cell lymphotropic virus type 1 (HTLV-1) is an important biomarker associated with the monitoring and risk stratification of adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). However, the lack of standardized quantification methods limits its broader application. This study evaluated a novel absolute quantification approach based on whole blood volume and compared its performance with established protocols. A total of 66 HTLV-1-infected individuals were analyzed using six qPCR-based methodologies, including volumetric quantification (copies/µL) by absolute quantification and the Tamegão-Lopes method, as well as normalization per 1000 cells (whole blood, buffy coat, PBMCs, and CD4+ T cells). Association and agreement were assessed using Pearson’s correlation, Bland–Altman analysis, concordance correlation coefficients (CCCs), and Deming regression. Absolute quantification showed strong correlation with both the Tamegão-Lopes method and CD4+-based quantification (r = 0.93 and 0.84, respectively; p < 0.001) and high agreement (CCC = 0.866 and 0.811, respectively), with modest systematic bias (−0.273 log10 copies/µL and 0.115 log10 copies/103 cells, respectively). Leukocyte-normalized methods showed greater discrepancies, likely due to dilution by uninfected cells. These findings show that quantification based on total blood volume is a simplified, operationally feasible alternative for assessing HTLV-1 proviral load.