Antithrombin III (Functional Activity) — Plasma
Antithrombin (AT) is a major endogenous anticoagulant. Functional activity assays help diagnose hereditary or acquired AT deficiency, which is associated with increased venous thromboembolism risk.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| % activity | 70 – 130 | 89 – 114 | 88 – 112 | 82 – 116 |
In normal pregnancy, AT activity remains within or slightly below the standard range, with mild physiologic decreases most noticeable in the 3rd trimester and early postpartum.
Causes of Low Antithrombin III
- Pregnancy-related / physiologic
- Mild decrease in late pregnancy
- Preeclampsia, severe preeclampsia, HELLP
- Placental abruption, IUFD with DIC
- Sepsis or severe infection
- Heparin therapy (especially unfractionated heparin)
- Congenital antithrombin deficiency
- Type I: decreased antigen + activity (typically 40–60%)
- Type II: normal antigen, reduced activity (qualitative defect)
- Best tested postpartum or outside pregnancy
- Acquired (non–pregnancy specific)
- Liver disease
- Nephrotic syndrome
- Protein-losing enteropathy
- Disseminated intravascular coagulation
- Major surgery, trauma, ECMO
- Malignancy or severe systemic inflammation
- When results may be unreliable
- Acute thrombosis
- During heparin therapy
- Late pregnancy / postpartum
- Severe illness or sepsis
Marked reductions should prompt repeat testing when clinically stable and off heparin; family studies may help confirm hereditary deficiency.
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–31.
- Wallach J. Interpretation of Diagnostic Tests, 8th ed.
- Fischbach FT, Dunning MB. A Manual of Laboratory and Diagnostic Tests, 7th ed.
- Reviews on hereditary/acquired antithrombin deficiency and thrombophilia management.