Laboratory diagnosis of congenital thrombophilia: current and past practice in Australia

Abstract number: P1018

Favaloro^* E. J., Bonar† R., Wheeler‡ M., Low§ J., Aboud¶ M., Duncan†† E., Smith^** J., Exner^** T., Lloyd†† J., Marsden† K.

^*Haematology ICPMR Westmead Hospital, Australia; ^**ICPMR Westmead Hospital, Australia ††IMVS Adelaide, Australia; ‡Monash Medical Center, Australia; †RCPA QAP, ICPMR Westmead Hospital, Australia; ¶Royal North Shore Hospital, Australia; §St Vincents Hospital, Australia;

We have conducted a series of multilaboratory surveys (n = 5) over the past five years, using a total of 15 plasma samples, in order to evaluate testing proficiency in the detection of congenital thrombophilia. Participant laboratories (n = 40–58) derived primarily from Australia and South-east Asia. Tests evaluated comprise the phenotypic assays Protein C (PC), Protein S (PS), anti-thrombin III (ATIII), and Activated Protein C Resistance (APCR). Genotypic tests are covered by another component of the haematology QAP. Most laboratories performed PC using chromogenic (approximately 75%), or clot based (approximately 15%) assays, with few (<10%) performing antigenic assessments. PS is most often assessed (~60%) by immunologic/antigenic assays (previously ELISA or EID; currently ELISA or LIA), usually of free PS and less often of total PS, or by functional/clot based assays (approximately 40%). ATIII is usually assessed by functional chromogenic assays (approximately 95%), with few (<5%) performing antigenic assays. APCR is usually assessed using APTT (approximately 50%) or RVVT (approximately 50%) clot-based assays. The APTT APCR is typically performed using factor V deficient plasma predilution, whereas the RVVT APCR is typically performed without. Laboratories using the RVVT procedure appear to perform better in detection of APCR, with the APTT method group more likely to yield occasional false negative and false positive findings (approximately 5% occasions). Some clot-based PC and PS assays still appear to be influenced by APCR status, and yield lower apparent PC and PS levels on these occasions. The overall ‘error rate’ for PC, PS and ATIII is also around 5% (i.e. false ‘normal’ interpretations for deficient plasma or false ‘abnormal’ interpretations for normal plasma). The coefficients of variation (CVs) for assays range from 5 to 40%, with highest CVs typically obtained with PS assays. Results of our Australian surveys are compared with those of other international survey groups, such as the European-based ECAT program.