Erroneously low APC ratio in patients with lupus anticoagulant

Temporal Changes in the Activated Protein C Resistance Ratio in a Heterozygote for Factor V Leiden Are Abolished by Dilution in Factor V-Deficient Plasma

Activated protein C resistance (APCR) is the single most common cause of hereditary thrombophilia and is caused by a mutation in the factor V molecule (FV Leiden). Screening for this defect has become a standard part of the laboratory evaluation of thrombophilia. However, the original APTT-based screening test does not discriminate completely between normal individuals and heterozygotes for FV Leiden. Diluting the patient's plasma in factor V-deficient plasma greatly improves the predictive ability of this test for the FV Leiden mutation. In this report we show that a heterozygote for FV Leiden (confirmed by DNA analysis) showed marked temporal variation in APCR ratio when tested by the original coagulation-based technique. Specifically, this individual tested normal in five of seven occasions tested (APCR ratio range 1.98-2.53; normal >2.0). Using the modified method, although some temporal variation remained, the diagnosis of APCR would have been made on every occasion (APCR ratio 1.85-2.12; normal >2.3). These findings constitute a further reason to use the modified procedure for APCR screening. Key Words: Thrombosis-Activated protein C resistance— Laboratory evaluation.

Factor V:Q506 mutation and anticardiolipin antibodies in systemic lupus erythematosus

Inherited resistance to activated protein C (APC resistance) is an important risk factor of venous thrombosis. It is caused by a point mutation in the gene coding for coagulation factor V, called FV:Q506. Arterio-venous thrombosis is a common and serious medical problem in patients with systemic lupus erythematosus (SLE). We studied the prevalence of the factor V mutation associated with APC resistance and IgG anticardiolipin antibodies (aCLs) in an epidemiological cohort of 78 Swedish SLE patients, to determine their roles as risk factors for thrombosis. In addition, a detailed evaluation of the clinical manifestations in these patients was performed. Totally, 19 (24%) of the 78 SLE patients had thrombosis, 11 (14%) had venous thrombosis and 8 (10%) had a cerebral infarction caused by occlusion of cerebral vessels. Twenty-six (33%) SLE patients were aCL positive and 8 (10%) were heterozygous for the factor V mutation. Only one of the patients with venous thrombosis and one of the patients with cerebral thrombosis had the FV:Q506 mutation, whereas 3 patients with venous thrombosis and 5 patients with cerebral infarction were aCL positive. Eleven of 19 patients with heart valve disease were aCL positive, a statistically significant association (P = 0.01). In conclusion, we found no statistically significant association between venous thrombosis and FV:Q506 mutation or venous thrombosis and aCL positivity. There was, however, an association between heart valve disease and aCL positivity.

Anticardiolipin antibodies do not seem to be associated with APC resistance in vivo or in vitro

Anticardiolipin antibodies (aCL) or lupus anticoagulants (LA) have been found to exert an inhibitory action upon the activation and function of protein C, a natural coagulation inhibitor. Recently an in vitro phenomenon called resistance to activated protein C (APC resistance) has been described as the most frequent cause of hereditary thrombophilia. In order to see whether a positive association of APC resistance with aCL exists we examined plasma of 162 consecutive outpatients referred for thrombophilia screening. Further, the IgG fraction was isolated from plasma of two aCL-positive and LA-negative patients and of two aCL-negative healthy subjects by means of protein A affinity chromatography. Each of these isolates was mixed with normal plasma, and the APC resistance was assayed; 25/162 (15.4%) patients had confirmed abnormal APC resistance. Only 1/25 (4.0%) APC resistance-positive patients and 11/137 (8.0%) APC resistance-negative patients had positive IgG- and/or IgM-aCL (p = 0.5, nonsignificant). In the in vitro test system the APC resistance ratio remained unaffected after addition of normal IgG or aCL-IgG fraction in the tested normal plasma and did not deviate from the range of buffer controls. These data do not suggest any association of aCL with abnormal APC resistance. aCL-IgG fractions from aCL-positive and LA-negative plasmas do not interfere with the APC resistance test system in vitro in low concentration.