Aspirin resistance and compliance with therapy.
BACKGROUND
Aspirin resistance is associated with increased cardiovascular risk in aspirin-treated patients. Poor compliance may explain many cases of "resistance," yet few clinical studies have used objective measurement of therapy compliance. We did so in a case-controlled study.


METHODS
We enrolled patients within 24 h of ischemic stroke and a group of controls taking aspirin who had never suffered a vascular event on therapy. All claimed to be compliant. We assessed platelet function using platelet function analyser (PFA)-100 and rapid platelet function analyser (RPFA) devices, applying standard definitions of resistance. We used high-performance liquid chromatography for levels of aspirin metabolites in the urine to confirm compliance with therapy. We compared rates of resistance in stroke patients and controls, and performed subgroup analysis restricted to patients with objective confirmation of recent aspirin ingestion.


RESULTS
We recruited 90 cases and 90 controls. Complete platelet function tests were available in 177. Resistance rates seen in cases and controls, respectively, were: resistance on one or more test, 30 (34%) versus 21 (25%), P= 0.19; on PFA-100 testing only, 28 (32%) versus 15 (18%), P= 0.031; on RPFA testing only, 16 (18%) versus 12 (14%), P= 0.54; resistance on both tests, 12 (14%) versus 5 (6%), P= 0.037. When only patients with objective evidence of recent aspirin ingestion were considered (n = 71), rates were similar regardless of definition of resistance used.


CONCLUSIONS
Aspirin resistance is common but poor compliance accounted for nearly half of cases of apparent aspirin "failure." Objective measures to assess compliance are essential in studies of aspirin resistance.
Dawson
Jesse
J
Acute Stroke Unit, Division of Cardiovascular and Medical Sciences, Faculty of Medicine, University of Glasgow, Western Infirmary, Glasgow, G11 6NT, UK. j.dawson@clinmed.gla.ac.uk
Quinn
Terry
T
Rafferty
Mark
M
Higgins
Peter
P
Ray
Gautamanda
G
Lees
Kennedy R
KR
Walters
Matthew R
MR
D000368:Aged; D000369:Aged, 80 and over; D001241:Aspirin / Q000627:therapeutic use* / Q000652:urine; D015415:Biomarkers / Q000652:urine; D001792:Blood Platelets / Q000187:drug effects*; D002545:Brain Ischemia / Q000097:blood / Q000188:drug therapy* / Q000652:urine; D016022:Case-Control Studies; D016009:Chi-Square Distribution; D002851:Chromatography, High Pressure Liquid; D004351:Drug Resistance; D005260:Female; D006801:Humans; D008297:Male; D055118:Medication Adherence; D008875:Middle Aged; D010975:Platelet Aggregation Inhibitors / Q000627:therapeutic use* / Q000652:urine; D010979:Platelet Function Tests; D011237:Predictive Value of Tests; D011446:Prospective Studies; D012606:Scotland; D020521:Stroke / Q000097:blood / Q000188:drug therapy* / Q000652:urine; D017211:Treatment Failure
D015415:Biomarkers; D010975:Platelet Aggregation Inhibitors; D001241:Aspirin