<prompt> A35 yr old lady with normal PT and increased aPTT. 2 year back, she was operated for cholecystectomy did not have any bleeding episoe. What is next investigation for clinical diagnosis -Factor VIII assayAnti viper venom assayPlatelet aggregation testRistocetin Cofactor assay  <response>: The dilute Russell viper venom test is a commonly used screening test for LA. It is called the dilute Russell viper venom time test because the phospholipid reagent is diluted such that its concentration becomes rate limiting; thus, it becomes a sensitive assay Ref Davidson 23rd edition pg 283 