PB1154 - Temporary isolated reagent-specific gross prolongation of APTT can be caused by acidosis

Background: Unexplained isolated prolonged APTTs may need investigation to confirm presence or absence of clinically relevant defects. In our hospital group a sample with isolated unexplained prolonged APTT of ≥ 10 seconds may be transferred between sites for further investigation We noticed an in vitro shortening of APTT in such a sample from a patient with diabetic keto acidosis (DKA) and a reduced in vivo pH and decided to investigate.

Aims: To assess impact of in vivo pH on APTT.

Methods: We identified 18 patients with an isolated unexplained prolonged APTT with Actin FS >10 seconds above the reference range who had an in vivo pH of < 7.3 ( normal range 7.32-7.75) . We measured FVIII, FIX FXI FXII in all cases and APTTs with Synthasil in 13 /18 .

Results: In all 18 cases the VIII, IX, XI and XII levels were > 50 IU/dl. APTT with SynthASil was within the reference range in all 13 cases assessed. We observed pH in vivo and compared this to APTT with AFS over time in 10 patients with DKA (Figure 1). All had initial in vivo pH < 7.0 and all showed normalisation of APTT as in vivo pH recovered to ≥ 7.2 within 48 hours. In 7 patients with an isolated, prolonged APTT (Actin FS) and in vivo pH ≤7.1 we found that the APTT decreased in vitro over 2 hours after initial testing by a median of 4.7 seconds with Actin FS and 1.6 seconds with SynthASil.

Conclusion(s): Acidosis in vivo can cause an acquired, gross, transient prolongation of APTT if determined with Actin FS. APTT with SynthASil was normal in all cases where this was determined, negating the need for factor assays. The APTT with AFS recovered to normal when pH recovered to >7.2.