Several methods may be used: mixing studies may be performed to determine a deficiency in a given factor or identify the presence of circulating anticoagulants. A mixing study is normally conducted with an APTT; since the thromboplastin reagents used for determination of Quick's time are generally less sensitive to the presence of circulating anticoagulants, mixing studies performed with Quick's time may in fact provide little useful information.
Correction or non-correction of APTT is determined by calculating the Rosner index (RI) as follows:
RI = [(APTT mixed - APTT standard) / APTT patient] x 100
APTT is considered to be corrected where the Rosner index is below 12%. It is not corrected if the Rosner index is above 15%. There is an area of uncertainty between 12 and 15%.
Correction of APTT during the mixture test suggests coagulation factor deficiency and points to the need for factor measurement.
Non-correction of APTT suggests the presence of circulating anticoagulant. While the presence of circulating lupus anticoagulant does not normally result in increased haemorrhagic risk, the presence of this type of anticoagulant may be concomitant with, and indeed mask, a coagulation factor deficiency.
Consequently, non-correction of APTT during mixing studies requires screening for factor deficiency in order to rule out haemorrhagic risk.
For lengthening of Quick's time alone, determination of factor VII should be performed. This procedure may be carried out using a single dilution of the plasma to be tested since potential interference due to the presence of circulating lupus anticoagulant is restricted because of the relatively low sensitivity of thromboplastins to such anticoagulant. However, this test may be performed on several dilutions of the plasma to be examined in order to definitively rule out any interference. Factor VII deficiencies may or may not be detected depending on the thromboplastin used to determine Quick's time, since certain thromboplastins are insensitive to factor VII deficiency.
For lengthening of APTT alone, determination of factors VIII, IX, XI and XII should be carried out. Measurement using several dilutions (buffered) of test plasma can be used to counter the presence of circulating lupus anticoagulant that might interfere with measurement and result in erroneous interpretation where testing is performed on only one dilution of the plasma to be examined.
Lengthening of Quick's time and of APTT requires measurement of common pathway factors I (fibrinogen), II, V and X. As with factor VII, assay of factors II, V and X may be performed using either a single dilution or several dilutions of the plasma to be examined.