Prolonged APTT readings render which screening test invalid?

Prolonged activated partial thromboplastin time (APTT) readings can significantly affect the accuracy of the Activated Protein C Resistance (APCR) screening test. APCR is a test that assesses the function of protein C, a key player in the blood coagulation process. When APTT is prolonged, it suggests an alteration in the intrinsic pathway of coagulation which can mask the underlying deficiency related to protein C or indicate the presence of other coagulation factors.

In this context, a prolonged APTT can lead to misleading results in the APCR screening, as it does not accurately reflect the functionality of protein C in the presence of disorders affecting the intrinsic pathway. If there's a problem with the intrinsic coagulation factors, it may result in false negatives or positives, making it an unreliable screening test under these circumstances.

The other tests mentioned are not directly influenced by APTT prolongation to the same degree or are not specific screenings that hinge critically on APTT values. For instance, a protein S assay can be performed independently of the APTT, and liver function tests or complete blood counts assess other physiological aspects that are not reliant on the APTT variability. Thus, the relationship between APTT and APCR screening is critical