Relationship Between aPTT and ACT Values
There is no reliable direct conversion formula between activated Partial Thromboplastin Time (aPTT) and Activated Clotting Time (ACT) due to their poor correlation and weak agreement in clinical settings. 1
Understanding aPTT and ACT Tests
aPTT Test
- Used to measure the effect of various anticoagulants on the coagulation system, including direct thrombin inhibitors and unfractionated heparin (UFH) 1
- Laboratory-based test typically reported in seconds
- Normal range varies by laboratory but typically 25-35 seconds
- Therapeutic range often 1.5-2.5 times normal baseline value
ACT Test
- Point-of-care test that measures time (in seconds) required for whole blood to clot when an activating agent is added 1
- Used primarily during procedures requiring high-dose anticoagulation such as cardiopulmonary bypass (CPB) and ECMO
- Normal range typically 90-130 seconds
- Therapeutic ranges vary by procedure:
- 180-220 seconds for ECMO
300 seconds for cardiopulmonary bypass
Evidence Against Direct Conversion
Multiple studies demonstrate poor correlation between aPTT and ACT values:
- In ECMO patients, Yie et al. found only a weak correlation between ACT and aPTT (r=0.177; p=.037) in 315 paired samples 1
- Retrospective studies confirmed only a moderate degree of positive correlation between aPTT ratio and normalized ACT (Pearson correlation coefficient r=0.55) 1
- Studies demonstrate poor correlation of ACT with aPTT or anti-Xa levels in patients receiving UFH during CPB or ECMO 1
Factors Affecting Correlation
The relationship between ACT and aPTT is affected by multiple variables:
- Platelet count and function (thrombocytopenia significantly affects concordance) 2
- Blood urea levels 2
- Recent blood product transfusions 1
- Hemodilution
- Presence of acute phase proteins
- Hemolysis and hyperbilirubinemia (common in ECMO) 1
- Patient-specific factors including age (infants and children have different baseline values than adults) 3
Clinical Implications
- For routine heparin monitoring, aPTT is generally preferred except in high-dose settings 4
- For high-dose anticoagulation (CPB, ECMO), ACT is more commonly used 4
- When transitioning between monitoring methods, direct conversion is not reliable and may lead to inappropriate anticoagulation management
- In specialized settings, some clinicians have developed the "ACT differential" (difference between standard ACT and heparinase ACT) which showed better correlation with aPTT (r=0.74) than standard ACT alone (r=0.24) 5
Best Practice Recommendations
- Use the appropriate test for the clinical scenario rather than attempting conversion
- For standard heparin therapy, use aPTT with target of approximately twice control value (60-70 seconds) 6
- For high-dose heparin therapy, use ACT with procedure-specific targets
- When transitioning between monitoring methods, overlap measurements and titrate therapy based on the new test rather than attempting conversion
- Consider anti-Xa assays for more accurate heparin monitoring, especially in ECMO patients 1
The lack of reliable conversion between aPTT and ACT highlights the importance of using the appropriate test for the specific clinical scenario and understanding the limitations of each monitoring method.