Rapid responses are electronic comments to the editor. They enable our users to debate issues raised in articles published on bmj.com. A rapid response is first posted online. If you need the URL (web address) of an individual response, simply click on the response headline and copy the URL from the browser window. A proportion of responses will, after editing, be published online and in the print journal as letters, which are indexed in PubMed. Rapid responses are not indexed in PubMed and they are not journal articles. The BMJ reserves the right to remove responses which are being wilfully misrepresented as published articles.
The author states that: "Positive predictive values are similar in
both high and low risk groups, so 20% of women with an abnormal Doppler
result will develop pre-eclampsia." This statement seems to violate Bayes'
theorem that positive predictive value increases as pretest probability
increases. Do other readers understand how this is possible?