Antithrombin III Deficiency Workup

Updated: Jan 10, 2022
  • Author: James L Harper, MD; Chief Editor: Hassan M Yaish, MD  more...
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Laboratory Studies

Specific laboratory workup for suspected antithrombin III (ATIII) deficiency depends on the clinical setting. A study by Baiges et al indicated that clinical screening that assesses only anti–factor Xa activity may lead to underdiagnosis of antithrombin deficiency. The report included 89 patients with splanchnic vein thrombosis in whom, via classical diagnostic methods, no antithrombin deficiency was found. However, through functional and immunologic techniques (including enzyme-linked immunosorbent assay [ELISA], crossed immunoelectrophoresis, and western blot), as well as SERPINC1 sequencing, four of the patients (4.5%) were found to have antithrombin alterations, including two with SERPINC1 mutations, one with a variant resulting in transient antithrombin deficiency, and one with a congenital disorder of the N-glycosylation pathway. [18]

Antithrombin assays

Antithrombin III activity should be measured first.

If low, then antithrombin antigen is measured to look for mutations consistent with type II disease.

The single most commonly used test for antithrombin III assay is thrombin-heparin cofactor level.

Avoid measuring antithrombin III while the patient is on heparin or even warfarin.

Prothrombin time (PT) and activated partial thromboplastin time (aPTT)

These studies allow evaluation of the presence of inappropriate activation of the coagulation system.

aPTT is a useful screen for antiphospholipid (AP) antibody syndrome.

aPTT-mixing study may distinguish between AP antibody syndrome and disseminated intravascular coagulation (DIC). Advanced DIC may present with a persistently prolonged aPTT if fibrin degradation products inhibit fibrin generation or acquired deficiencies of coagulation factors are severe.

Protein C (antigen and activity tests) and protein S (total and free tests)

Protein C or protein S deficiencies are both associated with venous thrombosis and are important exclusions in evaluating congenital deficiency of antithrombin III.

In the newborn, protein S activity must be measured (in addition to total) because, whereas total antigen levels are lower in neonates than in adults, protein S activity is usually normal because of the lack of expression of C4-binding protein in the neonate. (C4 acts to bind protein S in children and adults.)

These tests may also be important in the acquired state to determine the extent of a given patient's thrombotic risk.

Factor V Leiden testing

The most common congenital procoagulant disorder, factor V Leiden, occurs in about 5% of patients and needs to be documented when attempting to make the diagnosis of congenital antithrombin III deficiency. Knowing what this level is also helps to define a given patient's procoagulant risk.

Although factor V Leiden does not commonly produce thrombosis during childhood, it may contribute to thrombosis started by other etiologies (eg, central venous catheters).

Physicians should note that this is not a measurement of factor V activity, but rather a determination of a specific mutation of factor V that leads to a decreased sensitivity to the inhibitory effects of protein C.

Homocysteine level

Increased levels of homocysteine are associated with an increased risk of thrombosis in adults, but this is rarely seen in children. In a child with elevated homocysteine levels, MTHFR gene analysis should be performed.

Anticardiolipin antibodies (both immunoglobulin G [IgG] and immunoglobulin M [IgM] class)

These should be measured by enzyme-linked immunoabsorbent assay (ELISA) or other physical means to rule out coexisting thrombotic risk from this source.


Imaging Studies


This should be performed in all patients with antithrombin III deficiency, especially if they have evidence of arterial thrombus.

Arterial thrombosis due to antithrombin III deficiency is uncommon.

Venous clots may migrate to arterial circulation through a patent foramen ovale or other communicating congenital heart defect (eg, atrial septal defect, ventricular septal defect, truncus arteriosus).

Doppler ultrasonography

Doppler ultrasonography of the affected extremity with compression should be performed at diagnosis and then used in follow-up to determine resolution of an acute thrombus. Venography or MR angiography may be needed for upper extremity thrombi.

Ventilation-perfusion scanning

Pulmonary thrombosis can be imaged with ventilation-perfusion scan.

Thin-cut spiral CT scanning has also been used for this, but small lesions may be missed.

MR angiography may be used to detect PE or other intrathoracic thrombi.



Given the significant risk of venous thrombosis associated with central venous line (CVL) placement in children without signs of antithrombin III deficiency, those children known to have a congenital antithrombin III deficiency should have CVLs placed only if significant need outweighs increased potential risk of a clinically significant clot.