PT and INR Interference by the Antibiotic Daptomycin in Commercial Thromboplastin Reagents: Dependence on Phospholipid Composition.

The antibiotic, daptomycin (brand name Cubicin, Cubist Pharmaceuticals) is approved in the US for treatment of complicated skin and skin structure infections caused by gram-positive pathogens (e.g., MRSA, methicillin-resistant Staphylococcus aureus). Recently, this antibiotic has been shown to interfere with prothrombin time (PT) assays, resulting in a prolongation of clotting times and artificial increases in the INR (International Normalized Ratio). The level of interference varies with Daptomycin concentration in the plasma, the brand of prothrombin time reagent as well as the type (normal or abnormal) of patient sample being analyzed. The bactericidal action of the antibiotic is known and is through a depolarization of bacterial membranes in the presence of physiologic calcium (Silverman, et al, Antimicrobial Agents and Chemotherapy, Aug 2003); therefore, daptomycin interference with the PT clotting assay is likely mediated through the phospholipid component of the thromboplastin formulations. To examine the dependence of daptomycin interference for a number of commercial PT formulations, testing was performed with daptomycin spiked samples of normal pooled plasma as well as abnormal samples from oral anticoagulant therapy (OAT) patients. For spiked normal samples, a small degree of prolongation in clotting times was observed with most PT reagents, with one reagent showing a significant prolongation. In OAT samples spiked with Daptomycin, the PT prolongation was greater for all formulations, with the one reagent again showing greater prolongation as for the normals. Ultimately, this variation in response was attributed to differences in phospholipid composition among the reagents, supported by previous studies of daptomycin with neutral and acidic phospholipids in the presence of calcium (D. Jung, et al., Chemistry and Biology, Jul 2004). A four-dimensional experimental design matrix was constructed using a mixture of four synthetic phospholipids (DOPC, DOPS, DOPE and DOPG), and the relationship of daptomycin-PT interference to percent composition of the phospholipids was examined. In both normal and OAT plasmas spiked with daptomycin, the major contributor to PT prolongation was the concentration of DOPG (phosphatidylglycerol) in the lipid mixture, with additional contributions correlated with DOPC (phosphatidylcholine). Because of the ubiquitous use of these phospholipids in commercial PT formulations, this work suggests that daptomycin interference should be assessed for each combination of PT reagent and INR range used.