We have conducted a series of multilaboratory surveys during the last 6 years to evaluate testing proficiency in the detection of congenital and acquired thrombophilia. For lupus anticoagulant (LA) testing, participant laboratories used a panel of tests, including activated partial thromboplastin time (aPTT; 100% of laboratories), kaolin clotting time (26 to 70%), and Russell's viper venom time (RVVT; 75 to 100%). Coefficients of variation (CVs) for assays ranged from 5 to 40%. RVVT assays appeared to be most sensitive and specific for detection of LA (fewer false-negatives or -positives), although laboratories performed best when they used a panel of tests. For congenital thrombophilia, tests evaluated comprised protein C (PC), protein S (PS), antithrombin (AT), and activated protein C resistance (APCR). Most participant laboratories performed PC using chromogenic (approximately 75%), or clot based (approximately 15%) assays, with few (< 10%) performing antigenic assessments. PS was most often assessed (approximately 60%) by immunological or antigenic assays, usually of free PS, or by functional or clot-based assays (approximately 40%). AT is usually assessed by functional chromogenic assays (approximately 95%). APCR was assessed using aPTT (approximately 50%) or RVVT (approximately 50%) clot-based assays, with the aPTT APCR typically performed using factor V-deficient plasma predilution, but the RVVT APCR typically performed without. Laboratories using the RVVT APCR generally performed better in detection of factor V Leiden-associated APCR, with the aPTT method group yielding higher false-negative and/or false-positive findings (approximately 5% of occasions). Some clot-based PC and PS assays appeared to be influenced by APCR status, and yielded lower apparent PC and PS levels with positive APC resistance. The overall error rate for PC, PS, and AT was approximately 2 to 8% (i.e., false-normal interpretations for deficient plasma or false-abnormal interpretations for normal plasma). The CVs for these assays ranged from 5 to 40%, with highest CVs typically obtained with PS assays.