title: Interference of factor V Leiden on protein S activity: evaluation of a new prothrombin time-based assay.
abstract: Protein S activity in plasma from factor V Leiden (FVL)-positive patients may be lower than expected. We investigated a new commercially available method for protein S for such interference. Protein S activity was measured for plasmas from 50 individuals with FVL and their results were compared with those obtained for plasmas from 47 sex-matched and age-matched individuals without FVL. We assumed that the median protein S activity value from a relatively large number of individuals with or without FVL would not be significantly different if there is no influence from FVL. The FVL-positive plasmas gave relatively (albeit not significantly) lower protein S levels than FVL-negative plasmas when both were tested undiluted (86 versus 93 IU/dl, P = 0.06). Those differences were reduced (98 versus 102 IU/dl, P = 0.58) when testing was performed on diluted plasmas. Furthermore, the proportion of patients with FVL identified as low-abnormal on the basis of the specific cut-off values (undiluted = 64 U/dl; diluted = 71 IU/dl), which was 8% when testing was performed on undiluted plasmas, was reduced to 4% when testing was performed on diluted plasmas. Conversely, the corresponding proportions of patients without FVL remained unaltered (4.3 versus 4%). In conclusion, these results indicate that the evaluated method is somewhat affected by FVL and that dilution of plasma prior to testing improves specificity. Protein S activity measurement for FVL-positive patients should be performed on diluted plasma and the results interpreted on the basis of the cut-off value specifically determined for diluted plasmas.
authors.affiliation: Angelo Bianchi Bonomi Hemophilia and Thrombosis Center, Department of Internal Medicine and Medical Specialties, University and IRCCS Maggiore Hospital, Mangiagalli and Regina Elena Foundation, Milano, Italy. armando.tripodi@unimi.it
authors.forename: Armando
authors.initials: A
authors.lastname: Tripodi
authors.forename: Daniela
authors.initials: D
authors.lastname: Asti
authors.forename: Veena
authors.initials: V
authors.lastname: Chantarangkul
authors.forename: Eugenia
authors.initials: E
authors.lastname: Biguzzi
authors.forename: Pier Mannuccio
authors.initials: PM
authors.lastname: Mannucci
mesh_terms: D001780:Blood Coagulation Tests / Q000379:methods; D005165:Factor V / Q000737:chemistry / Q000378:metabolism*; D005260:Female; D006801:Humans; D008297:Male; D017293:Protein S / Q000032:analysis* / Q000378:metabolism*; D012016:Reference Values; D015203:Reproducibility of Results
chemical_list: D017293:Protein S; C095381:factor V Leiden; D005165:Factor V