Influence of comorbid depression and diagnostic workup on diagnosis of physical illness: a randomized experiment

Patients with mental illness are less likely to receive the same physical healthcare as those without mental illness and are less likely to be treated in accordance with established guidelines. This study employed a randomized experiment to investigate the influence of comorbid depression on diagnostic accuracy.

Physicians were presented with an interactive vignette describing a patient with a complex presentation of pernicious anemia. They were randomized to diagnose either a patient with or without (control) comorbid depression and related behaviors. All other clinical information was identical. Physicians recorded a differential diagnosis, ordered tests, and rated patient likeability.

Fifty-nine physicians completed the study. The patient with comorbid depression was less likeable than the control patient (p=0.03, 95 % CI [0.09, 1.53]). Diagnostic accuracy was lower in the depression compared to control condition (59.4 % vs. 40.7 %), however this difference was not statistically significant χ²(1)=2.035, p=0.15. Exploratory analyses revealed that patient condition (depression vs. control) interacted with the number of diagnostic tests ordered to predict diagnostic accuracy (OR=2.401, p=0.038). Accuracy was lower in the depression condition (vs. control) when physicians ordered fewer tests (1 SD below mean; OR=0.103, p=0.028), but there was no difference for physicians who ordered more tests (1 SD above mean; OR=2.042, p=0.396).

Comorbid depression and related behaviors lowered diagnostic accuracy when physicians ordered fewer tests - a time when more possibilities should have been considered. These findings underscore the critical need to develop interventions to reduce diagnostic error when treating vulnerable populations such as those with depression.