Assays used to monitor heparin
| Assay . | Assay details . | Advantages . | Disadvantages . | Comments . |
|---|---|---|---|---|
| ACT | Fresh whole blood + activator; initiates coagulation via the contact factor pathway | Rapid, point-of-care test | Nonspecific to heparin | Typical range for ECMO is 180-220 s |
| Most useful at very high concentrations of heparin when PTT is unmeasurable (cardiopulmonary bypass) | Prolonged ACT may be due to heparin; hypothermia; low concentrations of factor XII, XI, X, IX, V, II, or fibrinogen; thrombocytopenia; or platelet dysfunction | The type of activator influences the results | ||
| PTT | Citrated plasma + activator; initiates coagulation via contact factor pathway | Used for decades to measure anticoagulant effect of heparin in patients with deep vein thrombosis | Nonspecific to heparin | Therapeutic PTT range for adults with DVT is 1.5-2.5 times midpoint of normal PTT range |
| Widely available | Prolonged PTT may be due to heparin; low concentrations of factor XII, XI, X, IX, V, II, or fibrinogen; or lupus anticoagulant | |||
| PTT may be shortened because of elevated factor VIII or fibrinogen | ||||
| Anti-Xa | Citrated plasma is added to a known amount of excess FXa | Direct measure of heparin effect | Some assays are affected by hyperlipidemia and hyperbilirubinemia | Some assays add exogenous AT, others do not; this can influence the results |
| The heparin in the sample binds to AT and inhibits Xa | Not influenced by coagulopathy, thrombocytopenia, or dilution | May not be always available | Therapeutic range for adults with DVT: 0.3-0.7 U/mL | |
| Residual Xa cleaves a chromogenic substrate that is measured |
| Assay . | Assay details . | Advantages . | Disadvantages . | Comments . |
|---|---|---|---|---|
| ACT | Fresh whole blood + activator; initiates coagulation via the contact factor pathway | Rapid, point-of-care test | Nonspecific to heparin | Typical range for ECMO is 180-220 s |
| Most useful at very high concentrations of heparin when PTT is unmeasurable (cardiopulmonary bypass) | Prolonged ACT may be due to heparin; hypothermia; low concentrations of factor XII, XI, X, IX, V, II, or fibrinogen; thrombocytopenia; or platelet dysfunction | The type of activator influences the results | ||
| PTT | Citrated plasma + activator; initiates coagulation via contact factor pathway | Used for decades to measure anticoagulant effect of heparin in patients with deep vein thrombosis | Nonspecific to heparin | Therapeutic PTT range for adults with DVT is 1.5-2.5 times midpoint of normal PTT range |
| Widely available | Prolonged PTT may be due to heparin; low concentrations of factor XII, XI, X, IX, V, II, or fibrinogen; or lupus anticoagulant | |||
| PTT may be shortened because of elevated factor VIII or fibrinogen | ||||
| Anti-Xa | Citrated plasma is added to a known amount of excess FXa | Direct measure of heparin effect | Some assays are affected by hyperlipidemia and hyperbilirubinemia | Some assays add exogenous AT, others do not; this can influence the results |
| The heparin in the sample binds to AT and inhibits Xa | Not influenced by coagulopathy, thrombocytopenia, or dilution | May not be always available | Therapeutic range for adults with DVT: 0.3-0.7 U/mL | |
| Residual Xa cleaves a chromogenic substrate that is measured |
DVT, deep vein thrombosis; FXa, factor Xa.