MIRCA is a reproducible assay. (A-C) MIRCA OIs for individuals with HbSS were recorded longitudinally at routine clinic visits between November 2022 and July 2024. The NOI (n = 6) (A) and HOI (n = 5) (B) of steady-state individuals remained stable over time (P = .44, P = .13, Wilcoxon signed rank test) with a median percent point change of 13.3% and 15.7%, respectively. Four individuals (C) were not at steady state, defined as having had an acute VOE or ACS episode within the past 30 days, and were analyzed separately. The median NOI value of individuals with recent history of acute crisis rose by 101% compared with initial baseline measurement and median NOI increased by 8.6% (P = .12, Wilcoxon signed rank test). (D) CVs from comparable biomarkers (NOI vs EI Max and HOI vs EI Min) were compared using a nonparametric Mann-Whitney U test. Although NOI and EI Max were not significantly different in terms of reproducibility (P = .13), EI Min was found to have a 33.9% higher CV compared with HOI (P = .0095). Raw values used to calculate CVs can be found in supplemental Table 1. LoRRca EI Max (E) and EI Min (F) values in another 25 HbSS steady-state individuals were also recorded longitudinally at routine clinic visits, with median percent point changes of 15.7% and 82.1%, respectively. However, the intrapatient difference between MIRCA and LoRRca biomarker values between sample collection time was not significantly different from 0 using a 1-sample t test: NOI (P = .32), HOI (P = .09), EI Max (P = .15), and EI Min (P = .61). ∗∗P ≤ .01. ns, not significant.