Screening performance of quantitative blood loss for postpartum anemia among patients without baseline anemia

Quantitative blood loss (QBL) is widely used to identify postpartum hemorrhage. Although higher blood loss is expected to be associated with greater hemoglobin (Hb) 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 QBL in the postpartum assessment.

To evaluate the association between QBL and postpartum hematologic outcomes, including postpartum anemia and Hb decline, and to assess the diagnostic performance of commonly used QBL thresholds.

This retrospective cohort study included singleton deliveries at an urban safety-net hospital in 2023 with documented QBL and available baseline and postpartum Hb values. Patients with baseline anemia, transfusion before postpartum Hb measurement, or conditions expected to substantially alter bleeding risk or Hb levels independently of delivery blood loss were excluded. Multivariable logistic regression was used to evaluate associations between QBL and postpartum anemia, defined as Hb<10 g/dL, and multivariable linear regression was used to evaluate Hb decline. Diagnostic performance of QBL thresholds was assessed separately by delivery mode using sensitivity, specificity, positive predictive value, negative predictive value, and receiver operating characteristic analysis.

Among 769 deliveries, postpartum anemia occurred in 22.0% (169/769), and severe postpartum anemia, defined as Hb<8 g/dL, occurred in 0.5% (4/769). In adjusted models, compared with QBL<500 mL, higher QBL categories were associated with increased odds of postpartum anemia: 500 to 999 mL, adjusted odds ratio (aOR) 5.13 (95% confidence interval [CI], 2.59-10.13; P<.001), 1000 to 1499 mL, aOR 6.10 (95% CI, 1.94-19.23; P=.002), and ≥1500 mL, aOR 14.42 (95% CI, 2.26-91.95; P=.005). Each 500-mL increase in QBL was associated with greater Hb decline (adjusted mean difference, 0.53 g/dL; 95% CI, 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% CI, 0.571-0.721) for vaginal deliveries and 0.593 (95% CI, 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.

Among patients without baseline anemia, higher QBL was associated with postpartum anemia and greater Hb decline; however, QBL 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 QBL alongside baseline Hb, maternal symptoms, delivery characteristics, and clinical judgment.