Table 3 Main advantages and limitations of APTT and anti-Xa activity for UFH treatment laboratory monitoring

APTT

- largely available, low cost

- sensitive to clinically relevant changes of coagulation (coagulation proteins increases or deficiencies)

- numerous interferences; optical methods can be unreliable in case of DIC

- heparin sensitivity is highly reagent dependent

- APTT prolongation target needs to be established for each new batch of reagents

- APTT before UFH needed

- clinically irrelevant changes, or of dubious clinical relevance

APTT prolonged with:

• presence of antiphospholipid antibodies (viral infections)

• high CRP (depending on the reagent)

• high plasma levels in FDPs

• preanalytical (e.g., heparin/EDTA contamination, under-filling, delayed centrifugation, hypertriglyceridemia, hyperbilirubinemia)

APTT shortened with:

• preanalytical (e.g., prolonged venous stasis, vigorous mixing, coagulation of the sample, PF4)

• high factor VIII levels

Anti-Xa activity

- less vulnerable to biological interferences

- no requirement for measurement before UFH administration

- preanalytical interferences: PF4*; free hemoglobin and bilirubin if significant elevation

- insensitive to fluctuations in the underlying coagulation state (i.e., coagulation factor increases or defects), of potential clinical relevance

- AT deficiency (e.g. in sepsis): risk of heparin underdosing with kits containing exogenous AT; sensitivity to endogenous AT not evaluated with kits that do not contain exogenous AT

- variability in reagents composition (AT, dextran…)

- therapeutic range poorly defined

- not validated in hyperinflammatory states

- less available, more expensive