Identification of antithrombotic drugs related to total joint replacement using anonymised free-text notes: a search strategy in the Clinical Practice Research Datalink

OBJECTIVES: We aimed to design and test a method to extract information on antithrombotic therapy from anonymised free-text notes in the Clinical Practice Research Datalink (CPRD). SETTING: General practice database representative of the UK. PARTICIPANTS: All patients undergoing total hip replacement (THR, n=25 898) or total knee replacement (TKR, n=22 231) between January 2008 and October 2012 were included. Antithrombotic drug use related to THR or TKR was identified using anonymised free text and prescription data. PRIMARY AND SECONDARY OUTCOME MEASURES: Internal validity of our newly designed method was determined by calculating positive predictive values (PPVs) of hits for predefined keywords in a random sample of anonymised free-text notes. In order to determine potential detection bias, total joint replacement (TJR) patient characteristics were compared as per their status of exposure to antithrombotics. RESULTS: PPVs ranging between 97% and 99% for new oral anticoagulants (NOAC) or low-molecular weight heparins (LMWH) exposure related to TJR were obtained with our method. Our search strategy increased detection rates by 57%, yielding a total proportion of 18.5% of all THR and 18.6% of all TKR surgeries. Identified users of NOACs and LMWHs were largely similar with regards to age, sex, lifestyle, disease and drug history compared to patients without identified drug use. CONCLUSIONS: We have developed a useful method to identify additional exposure to NOACs or LMWHs with TJR surgery.