The activated partial thromboplastin time (APTT) assay is a basic hemostatic assay based on the time it takes for clots to form in plasma samples after the addition of calcium chloride. It is used to screen for various coagulation disorders. Several previous reports have suggested that magnesium (Mg) might contribute to coagulation reactions by binding to specific coagulation proteins. We investigated the effects of Mg on the APTT. In healthy controls, the APTT was significantly prolonged in proportion to the increase in the concentration of magnesium chloride in the range from 2.1 to 16.7 mmol/L. Among eight samples from patients with various disorders that exhibited prolonged APTT, two samples demonstrated shorter APTT when Mg was added, both of which were from patients that were positive for lupus anticoagulant. When we examined 206 clinical APTT samples, we found that Mg shortened the APTT of two samples. These two samples were also from lupus anticoagulant-positive patients (p-value: <0.003). Our findings regarding the unique effects of exogenous Mg on the APTT of lupus anticoagulant-positive patients might shed light on the role of Mg in APTT assays and lead to the development of a novel screening method for lupus anticoagulant.

IntroductionDetection of lupus anticoagulant (LA), an antiphospholipid (aPL) antibody, in a clotting time test is an important finding for diagnosis of antiphospholipid syndrome (APS). However, confirmation of LA requires several different testing procedures, some of which can be difficult and require time. We report here a simple and highly specific method for detecting LA.Materials and MethodsWe examined 66 plasma samples obtained from LA‐positive (LA) and 75 from LA‐negative (non‐LA) subjects, which included patients with acquired hemophilia and coagulation disorders, as well as from 43 healthy volunteer samples as normal controls. Activated partial thromboplastin time (APTT) was determined by adding 20 mmol of CaCl2 (Ca‐APTT) or 25 mmol of a mixture of Mg and Ca (Mg‐APTT). The ratio of Mg‐APTT/Ca‐APTT was then calculated and used as the Mg/Ca Index.ResultsThe Mg/Ca Index value for the LA group was significantly lower than that for the non‐LA and normal control groups (P < .0001). When the cutoff value of the Mg/Ca Index was less than 1.00, the sensitivity of LA determination using the Mg‐APTT assay was 80.3%, while specificity was 100%.ConclusionOur findings indicate that the present Mg‐APTT assay is a simple yet highly specific method for LA detection.