Validation of Non-Invasive Hemoglobin Measurement and Its Utility for Anemia Screening in Pregnancy: A Cross-Sectional Diagnostic Accuracy Study

Objective: To evaluate the agreement between non-invasive hemoglobin (SpHb) and venous complete blood count (CBC) in measurements of Hb in pregnant women and to compare the diagnostic performance in detecting anemia. Methods: A prospective cross-sectional study enrolled 660 Thai pregnant women, with 220 participants in each trimester. SpHb was measured at the thumb (tHb) and index finger (fHb) using the Masimo Rad-67, while venous CBC hemoglobin (CBC-Hb) served as the reference. Agreement between SpHb and CBC-Hb was evaluated using Bland–Altman analysis and the intraclass correlation coefficient (ICC). Potential factors influencing SpHb measurement accuracy were identified using generalized linear models, and the diagnostic performance of the resulting predictive models was subsequently evaluated. Results: The mean difference between SpHb and CBC-Hb was significant, with SpHb consistently overestimating hemoglobin concentrations across all trimesters. Consequently, SpHb demonstrated very low sensitivity (2.1% for tHb and 3.2% for fHb) but high specificity for detecting maternal anemia. Agreement with CBC-Hb was poor for absolute measurements (ICC = 0.221–0.278) but moderate for consistency (ICC = 0.615–0.650), indicating moderate precision but poor accuracy. Although optimized SpHb cut-offs substantially improved diagnostic performance, the two methods remained non-interchangeable. Measurement accuracy was significantly influenced by gestational trimester, maternal BMI, and perfusion index, underscoring the need for pregnancy- and ethnicity-specific calibration algorithms. Conclusions: Although adjustment models improved the diagnostic performance of SpHb for detecting maternal anemia, the substantial positive systematic bias and only moderate agreement suggest that SpHb should not replace CBC-Hb for hemoglobin measurement without further calibration. With these refinements, non-invasive Hb measurement may become a practical and effective screening approach in pregnancy.