Screening Performance of Quantitative Blood Loss for Postpartum Anemia Among Patients Without Baseline Anemia

Background Quantitative blood loss is widely used to identify postpartum hemorrhage. Although higher blood loss is expected to be associated with greater hemoglobin decline, the strength of this relationship and the diagnostic performance of commonly used thresholds for postpartum anemia are not well defined. Further evaluation is needed to define the utility and limitations of quantitative blood loss in the postpartum assessment. Objectives To evaluate the association between quantitative blood loss and postpartum hematologic outcomes, including postpartum anemia and hemoglobin decline, and to assess the diagnostic performance of commonly used quantitative blood loss thresholds. Study Design This retrospective cohort study included singleton deliveries at an urban safety-net hospital in 2023 with documented quantitative blood loss and available baseline and postpartum hemoglobin values. Patients with baseline anemia, transfusion before postpartum hemoglobin measurement, or conditions expected to substantially alter bleeding risk or hemoglobin levels independently of delivery blood loss were excluded. Multivariable logistic regression was used to evaluate associations between quantitative blood loss and postpartum anemia, defined as hemoglobin <10 g/dL, and multivariable linear regression was used to evaluate hemoglobin decline. Diagnostic performance of quantitative blood loss thresholds was assessed separately by delivery mode using sensitivity, specificity, positive predictive value, negative predictive value, and receiver operating characteristic analysis. Results Among 769 deliveries, postpartum anemia occurred in 22.0% (169/769), and severe postpartum anemia, defined as hemoglobin <8 g/dL, occurred in 0.5% (4/769). In adjusted models, compared with quantitative blood loss <500 mL, higher quantitative blood loss categories were associated with increased odds of postpartum anemia: 500–999 mL, adjusted odds ratio 5.13 (95% confidence interval, 2.59–10.13; p<.001), 1000–1499 mL, adjusted odds ratio 6.10 (95% confidence interval, 1.94–19.23; p=.002), and ≥1500 mL, adjusted odds ratio 14.42 (95% confidence interval, 2.26–91.95; p=.005). Each 500-mL increase in quantitative blood loss was associated with greater hemoglobin decline (adjusted mean difference, 0.53 g/dL; 95% confidence interval, 0.39–0.68; p<.001). Discrimination was modest in both delivery modes. The area under the receiver operating characteristic curve was 0.646 (95% confidence interval, 0.571–0.721) for vaginal deliveries and 0.593 (95% confidence interval, 0.523–0.663) for cesarean deliveries, with no significant difference between delivery modes (p=.313). At the ≥500-mL threshold, sensitivity was 22.4% (17/76) and specificity was 96.3% (394/409) for vaginal deliveries, compared with sensitivity of 90.3% (84/93) and specificity of 16.8% (32/191) for cesarean deliveries. Conclusions Among patients without baseline anemia, higher quantitative blood loss was associated with postpartum anemia and greater hemoglobin decline; however, quantitative blood loss alone had limited screening performance. Overall, the ≥500-mL threshold failed to identify 40.2% (68/169) of postpartum anemia cases, and its sensitivity and specificity varied markedly by delivery mode. Postpartum complete blood count testing decisions should be individualized and incorporate quantitative blood loss alongside baseline hemoglobin, maternal symptoms, delivery characteristics, and clinical judgment.