Bioequivalence of C-reactive protein in fingerprick blood and serum measured using the point-of-care LumiraDx test for tuberculosis diagnosis in exposed contacts

C-reactive protein (CRP) is recommended to screen people living with HIV (PLWH) for tuberculosis (TB). LumiraDx is a portable platform that uses fingerprick blood. How CRP compares in fingerprick blood and serum is unknown. CRP was measured in 1034 consecutively recruited contacts of people with TB using fresh fingerprick blood (LumiraDx at point-of-care) and stored (−80 °C) serum [LumiraDx and cobas C-Reactive Protein (Latex) High Sensitive (CRPHS) in laboratories]. Agreement was assessed using Lin's concordance correlation coefficient (CCC), Passing-Bablok (PB) regression, and Bland-Altman (BA) plots. Sensitivity and specificity for TB were evaluated in 156 contacts with microbiological reference standard information, namely culture, Xpert MTB/RIF Ultra, or both. Strong agreement [CCC = 0.85, PB slope − 0.27 (95% confidence interval − 0.82, 0.2), BA mean difference 1 (−1,3)] was observed between LumiraDx on fingerprick blood and serum. Similar agreement occurred for serum CRPHS vs. LumiraDx on serum [0.79; 1.1 (−1.1, 2.3); 11 (9, 14)] or fingerprick blood [0.75; 1.3 (−0.6, 2.5); 10 (8, 13)]. Areas under the receiver operating characteristic curves (AUROCs) were 0.747 (0.595, 0.899) for fingerprick LumiraDx, 0.761 (0.628, 0.893) for serum LumiraDx and 0.775 (0.636, 0.914) for serum CRPHS. At >5 mg/L, all tests showed identical sensitivity [77% (70, 83)]. Specificities were 60% (53, 68), 64% (57, 72) and 50% (43, 58), respectively. Serum storage duration did not affect performance. LumiraDx CRP readouts on fingerprick blood and serum correlate closely. Stored serum can be used for LumiraDx CRP measurement. High sensitivity methods increase the proportion of people who screen false-positive. • Our study evaluated whether stored serum can replace fingerprick blood when measuring C-reactive protein (CRP) to screen for tuberculosis (TB) with LumiraDx. We enrolled 1034 consecutively recruited TB contacts and compared LumiraDx fingerprick and serum results against a high sensitivity assay and a microbiological reference standard. LumiraDx fingerprick and serum showed strong agreement, and freezing duration did not affect performance. In the subgroup with reference standard data, all methods showed similar accuracy for TB screening. The findings support the use of existing serum samples and expand the practical utility of CRP testing.