4.5. Are the numbers of participants lost to follow-up implausible?
The reviewer should consider whether numbers of participants lost to follow-up are plausible. This may require domain knowledge, for example about the plausibility of little or no attrition given the context, condition, follow-up duration, and study protocol.
The reviewer should also consider the role of incentives to minimise attrition in the study and whether they could explain low rates of attrition.
It may be useful to consider what level of attrition was anticipated in the sample size calculation reported for the study. For example, if a substantial degree of attrition was anticipated, this may lead to concerns if there was actually little or no attrition in the study and no explanation is provided for this.
Round, equal numbers of participants lost to follow-up, or numbers lost to follow-up resulting in a perfect match with the planned sample size, may be suggestive of problems, but are unlikely to be sufficient to warrant concerns unless other problematic features are also present.
The answer to this check should contribute to a domain-level judgement.
Example
In a large multicentre trial of psychotherapy versus usual care for people with long-term depression, participants must attend the trial site every three months over an 18-month period to have their outcomes assessed. The trial manuscript reports that there was no loss to follow-up (all 524 participants at all study sites were retained in the study until the end of follow-up). The reviewer judges this to be very unusual for trials conducted in this population, where attrition rates are typically high even in trials with shorter follow-up durations. The reviewer answers “yes” for this check, and this response contributes to the domain-level judgement.