Purpose: Despite billions of doctor visits worldwide each year, little is known on whether doctors themselves affect patients' physical health after accounting for intervention and confounders such as patients' and doctors' data, hospital effects, nor how strong that doctors' effect is. Knowledge of surgeons' and psychotherapists' effects exists, but not for 102 other medical specialties notwithstanding the importance of such knowledge.
Methods: Eligibility Criteria: Randomized controlled trials (RCTs), case-control, and cohort studies including medical doctors except surgeons for any intervention, reporting the proportion of variance in patients' outcomes owing to the doctors (random effects), or the fixed effects of grading doctors by outcomes, after multivariate adjustment. Exclusions: studies of <15 doctors or solely reporting doctors' effects for known variables.
Sources: Medline, Embase, PsycINFO, inception to June 2020. Manual search for papers referring/referred to by resulting studies.
Risk of Bias: Using Newcastle-Ottawa scale.
Results: Despite all medical interventions bar surgery being eligible, only thirty cohort papers were found, covering 36,239 doctors, with 10 specialties, 21 interventions, 60 outcomes (17 unique). Studies reported doctors' effects by grading doctors from best to worst, or by diversely calculating the doctor-attributed percentage of patients' outcome variation, ie the intra-class correlation coefficient (ICC). Sixteen studies presented fixed effects, 18 random effects, and 3 another approach. No RCTs found. Thirteen studies reported exceptionally good and/or poor performers with confidence intervals wholly outside the average performance. ICC range 0 to 33%, mean 3.9%. Highly diverse reporting, meta-analysis therefore not applicable.
Conclusion: Doctors, on their own, can affect patients' physical health for many interventions and outcomes. Effects range from negligible to substantial, even after accounting for all known variables. Many published cohorts may reveal valuable information by reanalyzing their data for doctors' effects. Positive and negative doctor outliers appear regularly. Therefore, it can matter which doctor is chosen.