What is the normal baseline range for the APTT reaction? What types of conditions might cause a patient to give abnormal aPT time, and what can be done about it?
The normal range is approximately 25-40 seconds, although it will vary slightly depending on instrument. Although the 1:4 dilution with V deficient plasma strongly decreases interferences from oral anticoagulants and heparin therapy, it cannot be excluded that analysis from patients with high inhibitor activity such as those with phospholipid antibodies (i.e. lupus anticoagulant) may give an abnormal APTT. Increasing the dilution factor to 1:9 or 1:19 may correct the results. According to a study by Nowak et al, a 1:10 dilution may also be useful in children less than one year old. This is due to the special properties of the neonatal hemostatic system, such as low vitamin-K dependent coagulation factors and physiological prolongation of the PT and APTT.