Study subject to unmeasured confounders and biasesBMJ 2011; 342 doi: https://doi.org/10.1136/bmj.d3349 (Published 31 May 2011) Cite this as: BMJ 2011;342:d3349
- 1Wolfson Institute of Preventive Medicine, Queen Mary, University of London, London EC1M 6BQ, UK
- 2Newcastle Hospitals Community Health, New Croft Centre, Newcastle upon Tyne NE1 6ND, UK
The papers on venous thromboembolism in oral contraceptive users have similar faults to earlier database studies.1 2 These include lack of validation of venous thromboembolism (VTE) cases and lack of information on, or control of, important potential confounders such as duration of use, especially short duration use; family history; body mass index (BMI); and smoking.3 Jick and Hernandez’s study under-ascertained short duration use of oral contraceptives in cases. Also, women who used drospirenone—which is associated with an increased risk of VTE—were more likely to be short duration users than were users of older drugs. In many areas of the UK the prescription of drospirenone oral contraceptives is restricted because of their higher price. This results in a tendency for women at high risk of VTE to use these pills and may also account for the smaller numbers of drospirenone users and difficulty in finding enough controls. The annual rates of VTE in both studies (which were confined to oral contraceptive users)—1-3 per 10 000—were lower than those found in non-users of oral contraceptives in active surveillance studies,4 even when allowing for the restriction to idiopathic cases of VTE. Such major under-ascertainment would increase the likelihood of diagnostic and treatment bias among drospirenone users. In addition, confining studies to “idiopathic VTE” may fail to account for unmeasured confounders and biases and lead to analyses that are not representative of the population of oral contraceptive users at risk of VTE.5
The authors state that no evidence exists for non-contraceptive benefits of drospirenone oral contraceptives, even though they are licensed in the US for the treatment of severe premenstrual syndrome and acne.
Cite this as: BMJ 2011;342:d3349
Competing interests: AS and DM have received honorariums, conference sponsorship, and consultancy fees from drug companies, including Bayer HealthCare and MSD.