Laboratory tests in the clinical risk management of potential drug-drug interactions: a cross-sectional study using drug-dispensing data from 100 Dutch community pharmacies.

BACKGROUND: Patient safety and the life cycle of a drug are negatively influenced by the still increasing occurrence of potential drug-drug interactions (DDIs). Clinical risk management of potential DDIs is required in patients using drugs to influence the benefit-risk profile positively. Information about laboratory test results, in particular, may be useful in the assessment of potential DDIs for the individual patient.
OBJECTIVE: The objective of this study was to examine the frequency and nature of laboratory tests required for the assessment of the clinical relevance of potential DDIs in Dutch community pharmacies. In addition, the nature and clinical relevance of these potential DDIs is analysed.
METHODS: All patients from 100 Dutch community pharmacies using, according to dispensing information, two or more drugs concomitantly on a specified date (Wednesday, 4 April 2007), were included (n = 223,019). The anonymous dispensing data of the included patients were analysed against a list of DDIs requiring laboratory tests for the assessment of their clinical relevance. The number of patients at risk for these potential DDIs with severe adverse reactions was calculated. The frequency of potential DDIs requiring laboratory tests were stratified by age, sex and degree of polypharmacy.
RESULTS: Of the included patients, 24.4% had one or more potential DDIs (n = 54,427). In 9.0% of the included patients, one or more laboratory tests for the assessment of clinical relevance of the potential DDI were required (n = 19,968). The frequency of DDIs requiring laboratory tests increased with increasing age and number of drugs, but was not related to sex. The most commonly required laboratory tests were for renal function (42.2%), electrolytes (20.1%) and coagulation (13.1%). The percentage of patients at risk for potential DDIs requiring laboratory tests with adverse reaction category F (serious, irrecoverable disablement or death) was 2.5%; category E (increased risk of failure of life-saving therapy) was 0.6%; and category D (inconvenience with residual symptom and failure of therapy concerning serious but non-fatal diseases) was 3.8%.
CONCLUSIONS: A large number of patients in Dutch community pharmacies are at risk for potential DDIs requiring laboratory tests for the assessment of the clinical relevance of the interaction. There is a strong relationship between the frequency of DDIs requiring laboratory tests and age and the number of drugs concomitantly used. In the clinical risk management of potential DDIs, information about laboratory test results is of additional value. Future research is necessary in order to obtain more evidence on using laboratory tests in terms of which tests should be linked to pharmacy data, in which patients they should be done, how often and what actions should be taken when an abnormal value is found.