KM Sam^1,2, S Liu³, S Morgan³, J Clifford⁴, HG Schneider^1,5, QT Lam¹

1 Clinical Biochemistry Department, Alfred Pathology Service, Alfred Health, Melbourne, Australia; 
2 School of Clinical Science, Monash University, Clayton, Australia; 
3 Laboratory Haematology Department, Alfred Pathology Service, Alfred Health, Melbourne, Australia; 
4 Laboratory Haematology Department, Monash Pathology, Monash Health, Clayton, Australia; 
5 Department of Epidemiology and Preventative Medicine, Monash University, Melbourne, Australia;

Background

Hydroxocobalamin, the antidote of choice for cyanide toxicity, causes red dis-coloration of plasma reportedly interfering with laboratory tests.1,2 We report the findings from hydroxocobalamin spiking studies of plasma pools on two commonly used coagulation platforms.

Methods

Three patient-pools (PP) were prepared to test PT, INR, APTT, Fibrinogen, D-dimer and Anti-Xa concentrations. Each PP was spiked with 2 different hydroxocobalamin concentrations (high and low spike) and equal volume of coagulation buffers. The 15 samples (5 samples per PP) were tested using Stago STA-R Max (clot-based) and ACL Top750 (spectrophotometric) analysers.

Results

The buffer pools did not show significant dilution effect on both platforms. When spiked plasma pools results were assessed against the RCPAQAP APS (Analytical Performance Specifications), both methods showed mild degree of interference on PT (28% Stago, 22% ACL-Top) and INR (29% Stago, 22% ACL-Top). The rest of haemostasis parameters were acceptable on both platforms as per APS.

Discussion

Our study showed unexpected result of the clot-based method marginally more susceptible to hydroxocobalamin interference than the spectrophotometric method, which warrants further research to identify the interfering mechanism. Interval testing or checking on a different method is recommended in case of hydroxocobalamin interference where laboratory results are not clinically fitting.