Figure 1 gives a breakdown of the studies identified [12]. Searches identified 1990 records and, after the removal of duplicates, 1251 unique records were screened. Of these, 316 were sought for retrieval and 82 reports of 74 studies were found to be eligible for inclusion. Bibliographic citations for all included studies are provided in Additional file 3.

PRISMA flowchart detailing selection of studies for inclusion

Table 1 gives the study and population characteristics. Thirteen studies (17.6%) were identified as being conducted in rare conditions. Nineteen (25.7%) of the included articles were protocols for n-of-1 studies and the remainder (n = 55, 74.3%) reported on the results of completed studies. Fifty-four (25.7%) studies reported funding, 10 (13.5%) reported receiving no funding, and 10 (13.5%) did not report on whether they had received funding.

Studies were conducted in a range of countries including Australia (n = 9, 10.8%), Canada (n = 7, 9.5%), China (n = 10,13.5%), Netherlands (n = 9, 12.2%), United Kingdom (n = 6, 8.1%) and United States of America (n = 15, 20.3%).

They spanned a range of health and disease areas, with the most common being neurological conditions (n = 16, 21.6%). Six studies (8.1%) were not conducted in a specific disease area, e.g. blood transfusion-dependent patients.

Most of the studies included adult participants only (n = 60, 81.1%). Eight studies (10.8%) only included children (aged < 18 years old). Four studies (5.4%) included both adults and children. Studies were mostly of series of n-of-1 trials; 12 (16.2%) studies included a single participant only. The median (Q1, Q3) number of participants randomised was nine (4, 20) and the number of participants completing the studies was seven (3, 14). For the included protocols, the median (Q1, Q3) target sample size was 20 (10, 43).

The study and population characteristics were similar for studies conducted in rare and non-rare conditions, except for the number of participants involved (both randomised and completing) in the studies which was lower for the studies in rare conditions. A similar difference is seen in the target sample sizes stated in the study protocols.

Table 2 gives the design characteristics for the studies. The types of health technologies evaluated were: pharmaceutical (n = 46, 62.2%); nutritional (n = 4, 5.4%); medical device (n = 6, 8.1%); behavioural (n = 7, 9.5%); surgical (n = 1, 1.4%) and other types (n = 10, 13.5%). The comparators used were: placebos (n = 49, 66.2%); active treatments (n = 21, 28.4) and no intervention (n = 10, 13.4%). Studies most commonly compared two health technologies (n = 61, 82.4%). Nine studies (12.2%) compared three health technologies, three studies compared four health technologies (4.1%), and one study compared five health technologies (1.4%).

The median (Q1, Q3) number of periods was six (4, 8). The median (Q1, Q3) period length was 14 days (5, 28). The majority of studies blinded participants to treatment allocation (n = 57, 77.0%).

Washout periods were used in 32 studies (43.2%). The median (Q1, Q3) washout length was 7 days (2, 14). The median (Q1, Q3) total study duration was 77 days (42, 168).

A range of primary outcome measures were used: patient-reported outcome measure (PROM, n = 49, 66.2%); physiological parameter (n = 11, 14.9%); lab parameter (n = 1, 1.4%); clinical assessment (n = 3, 4.1%); behavioural test (n = 4, 5.4%); physical activity (n = 5, 6.8%); lab parameter and PROM (n = 1, 1.4%). The primary outcome was measured once per period in 32 studies (43.2%), multiple times per period in 40 studies (54.1%) and not reported in two studies (2.7%).

The design characteristics were mostly similar for studies in rare and non-rare conditions, however there were a few differences. None of the studies in rare conditions used a “no intervention” comparator, compared to ten (16.4%) of the studies in non-rare diseases. A higher incidence of blinding was identified in the studies in rare conditions (n = 12, 92.3%) compared to those in non-rare conditions (n = 45, 73.8%). The median period length was longer for those studies in rare conditions than for those in non-rare conditions (28 days compared to 14 days). Similarly, the median study length was longer for those studies in rare conditions than for those in non-rare conditions (119 days compared to 77 days). Washout periods were more commonly used in the studies in rare conditions (n = 8, 61.5%) compared to studies in non-rare conditions (n = 24, 39.3%).

Table 3 gives the analysis characteristics for the study. The included studies used a range of approaches for their analysis. Formal statistical approaches included: regression models (n = 17, 23.0%); t-tests (n = 17, 24.0%); Bayesian approaches (n = 11, 14.9%) and non-parametric analyses (n = 9, 12.2%). Graphs or visual inspection of data were used in four studies (5.4%). Ten studies stated they did not use formal analysis methods (15.4%) and methods of analysis were not reported in six studies (11.5%).

The studies defined “responders” to treatment in a variety of ways. The most common definitions were statistical significance with a p value < 0.05 (n = 15, 20.2%) and a specified numeric change in the outcome measure (n = 11, 14.9%). Almost half of the studies did not define what they would consider a responder (denoted as “Not given” in Table 3, n = 35, 47.3%).

The majority of studies combined the data from multiple n-of-1 trials or, in the case of study protocols, planned to do so (n = 48, 64.9%). There were 14 studies which involved multiple participants but did not combine the data across n-of-1 trials (18.9%). Combining data from multiple n-of-1 trials was not possible in trials including only one patient (n = 12, 16.2%).

Numerical results for individual participants were reported in 39 studies (52.7%) and P-values were reported in 27 studies (36.5%).

The analysis characteristics were similar for the studies in rare and non-rare conditions.