Comparison of influenza hemagglutination inhibition titers from capillary blood versus venipuncture

BACKGROUND: Capillary blood self-collection devices offer a minimally invasive alternative to venipuncture, potentially expanding access to serologic studies in outpatient and remote settings. We aimed to evaluate the concordance of influenza HAI titers between capillary blood collected using a minimally invasive device and venous blood collected via standard venipuncture. METHODS: We compared three paired samples collected from participants during a single visit: venipuncture, staff-assisted capillary device collection (clinic-collect), and participant self-collected capillary device with 72-hour delayed processing (self-collect) to simulate both on-site and at-home blood collection conditions. We compared HAI titers against influenza A(H1N1) antigen across collection methods using Pearson correlation coefficients, Bland-Altman plots, and paired t-tests. Sample volume adequacy and correlation thresholds for clinical interpretability were also assessed. RESULTS: Among 27 participants, HAI titers correlated strongly between venipuncture and both minimally invasive device approaches (clinic-collect: r = 0.955; self-collect: r = 0.937). Venipuncture titers were modestly higher, with mean differences of + 0.26 log₂ units (clinic-collect) and + 0.35 log₂ units (self-collect); these differences were below the 1-dilution (2-fold) threshold and not clinically meaningful. Most device samples (89-96%) fell within ±1 log₂ unit of venipuncture. On first attempt, 78% of device collections met the ≥ 600 μL volume threshold, increasing to > 93% with use of a second device. CONCLUSION: Minimally invasive devices yielded influenza HAI titers highly comparable to venipuncture, with clinically acceptable differences in adults. Self-collection with a minimally invasive device could serve as a practical, scalable alternative for serologic studies.