**4.2 Activated partial thromboplastin time**

Activated partial thromboplastin time (aPTT) is a plasma-based test, instead of a whole blood test, which only measures the initial 5% of thrombin generated and without incorporating platelet function or assessing clot strength. The aPTT measures the time from FXII activation to fibrin formation after addition of the PTT reagent and calcium [3, 19]. The therapeutic range for aPTT in adults has been shown to correlate to 1.5–2.5× the patient's baseline aPTT [22]. However, this range has not been validated in pediatric patients and can vary significantly compared to adults. For example, an aPTT that correlates to anti-factor Xa between 0.35 IU/ml and 0.7 IU/ml in a patient less than 1 year old is between 58 and 105 seconds, but in a patient 6–10 years old the anti-factor Xa correlates to an aPTT between 45 and 251 seconds [23]. The results of aPTT can be affected by fibrinogen level, presence of acute phase reactants, and increased levels of FVIII. These variables are often skewed in critically ill patients, which can lead to a high degree of intra- and interpatient variability [19].
