How is D-dimer (D-dimer test) interpreted in pregnancy?

D-dimer Interpretation in Pregnancy

D-dimer testing is not recommended to exclude pulmonary embolism in pregnancy due to its progressive physiological increase throughout normal pregnancy, resulting in poor specificity and inadequate negative likelihood ratio. 1

Physiological Changes in D-dimer During Pregnancy

• D-dimer levels increase progressively throughout normal pregnancy, with values rising by approximately 39% in each trimester compared to the previous one 1
• Reference intervals established for pregnancy by trimester are significantly higher than conventional cut-off points:
  • First trimester: 169-1202 µg/L 2
  • Second trimester: 393-3258 µg/L 2
  • Third trimester: 551-3333 µg/L (with 99% of pregnant women exceeding the conventional 500 µg/L cut-off) 2
• Twin pregnancies show even higher D-dimer values in the third trimester compared to singleton pregnancies (3.7 ± 2.5 vs 2.2 ± 1.6 µg/mL) 3

Diagnostic Limitations in Pregnancy

• D-dimer has only 73% sensitivity and 15% specificity for pulmonary embolism in pregnancy, with a negative likelihood ratio of 1.8 1
• Significant biological variation exists within individual pregnant women, with approximately half experiencing fluctuations of around 20 percentile points and 10% showing fluctuations exceeding 50 percentile points 4
• These fluctuations make repeated D-dimer measurements of limited clinical value in evaluating thromboembolic events during pregnancy 4

Clinical Approach to Suspected VTE in Pregnancy

• Despite limitations, a normal D-dimer value may still have some exclusion value for pulmonary embolism in early pregnancy (before 20 weeks) 1, 5
• If D-dimer is normal in early pregnancy, it may help avoid unnecessary radiation exposure to the fetus 1
• If D-dimer is elevated (which is expected in most pregnant women):
  • Proceed to bilateral compression ultrasonography of lower extremities 1
  • A positive ultrasound finding warrants anticoagulation without further imaging 1
  • If ultrasound is negative, continue with appropriate imaging studies 1

Postpartum Considerations

• D-dimer levels increase significantly in all women after delivery, often 1-10 times above the normal range 6
• Values are typically very high during labor and decrease quickly during the first 3 days postpartum 5
• Normalization takes several weeks, limiting D-dimer's utility in ruling out VTE until at least 4 weeks postpartum 5

Practical Recommendations

• When suspecting pulmonary embolism in pregnancy, do not rely solely on D-dimer to exclude the diagnosis 1
• Maintain a high index of suspicion as common symptoms like shortness of breath and rapid heart rate may be normal in pregnancy 1
• If using D-dimer, interpret results in the context of trimester-specific reference ranges rather than conventional cut-offs 2
• For D-dimer values above 3.2 µg/mL in the third trimester, consider closer monitoring as this threshold has been associated with higher rates of ultrasonographically detected thrombosis 3
• Always interpret D-dimer results in association with other clinical, laboratory, and imaging findings 6