Establishment of biological reference intervals for coagulation indicators in pregnancy using a non-parametric approach

This study aimed to evaluate changes in activated partial thromboplastin time (APTT), prothrombin time (PT), thrombin time (TT), and fibrinogen (FIB), and D-dimer (DD) levels during different trimesters of pregnancy in healthy pregnant women. The objective was to establish trimester-specific reference intervals for these coagulation parameters using the non-parametric percentile method as recommended by the Clinical and Laboratory Standards Institute (CLSI) C28-A3 guideline, thereby providing a basis for accurate clinical assessment of coagulation status during pregnancy. A total of 2,657 healthy pregnant women were selected from Sichuan JinxinXinan Women and Children Hospital from January 2024 to December 2024. Of these women, 646 were included in the first-trimester group; 1,128 were included in the second-trimester group; and 883 were included in the third-trimester group. In addition,198 healthy non-pregnant women were included in the control group. The coagulation indicators of each group were detected and analyzed, and differences in these indicators across different age groups were assessed. Following the CLSI C28-A3 guidelines, reference intervals were established using the non-parametric percentile method (P 2.5 -P 97.5 for APTT, PT, TT, and FIB; P 95 for DD). The established reference intervals were validated using an independent sample of 20 healthy pregnant women per trimester. No significant differences were observed in any coagulation indicators between pregnant women aged < 30 years and those aged ≥ 30 years in the first-, second-, and third-trimester groups( P > 0.01).APTT, PT, and TT decreased with the advancement of gestational age, and the differences between the periods of each gestation were statistically significant ( P < 0.01).FIB and DD levels increased with the advancement of gestational age, and the differences were statistically significant between each trimester( P < 0.01).The established reference intervals for the first trimester were: APTT 25.20–36.48 s, PT 11.10–13.00 s, TT 15.00–17.70 s, FIB 2.58–5.04 g/L, and DD ≤ 1.11 µg/mL. For the second trimester: APTT 24.40–34.56 s, PT 10.90–12.60 s, TT 14.70–17.20 s, FIB 2.95–5.27 g/L, and DD ≤ 2.94 µg/mL. For the third trimester: APTT 24.70–33.40 s, PT 10.60–12.40 s, TT 14.60–16.80 s, FIB 3.39–5.90 g/L, and DD ≤ 4.62 µg/mL. Compared with previously published reference intervals from other populations, our findings show similar trends but with notable differences in absolute values, particularly for FIB and DD, which may be attributed to geographical and ethnic variations. The biological reference intervals of all five coagulation indicators were successfully validated. Trimester-specific reference intervals were successfully established for five coagulation indicators using the non-parametric approach recommended by CLSI C28-A3. These intervals provide a crucial tool for accurate clinical diagnosis and prevention of coagulation disorders during pregnancy, potentially reducing misdiagnosis and unnecessary interventions associated with using non-pregnancy reference intervals. However, multicenter validation studies are needed to confirm the generalizability of these findings to other populations.