Do patients assess rectal bleeding accurately?

The accuracy of a patient's assessment of the volume of blood passed per rectum is unknown. The aim of the present study was to compare subjective estimations with known amounts of blood.

Thirty subjects (10 patients, 10 nurses and 10 doctors) made a visual estimation of four volumes of blood (0.25 mL, 10 mL, 50 mL, 200 mL), each placed in a standardized way into a toilet bowl. Subjects gave their estimations in mL, and also by drawing a line on a cup at the level to which they estimated that the amount of blood would fill the cup. Statistical analysis was carried out using Student's t-test. Multiple comparisons were analysed by anova.

There was no significant difference between the groups in their estimations of each of the four volumes. Each group made a statistically significant overestimation for the 0.25 mL, 10 mL and 50 mL volumes with cup marking, as well as with estimation in mL (except for patients with the 50 mL volume, where there was a non-significant overestimation). For the 200 mL volume, patients and doctors made statistically significant underestimations with cup marking, but for nurses the underestimation did not reach significance. For the 200 mL volume estimations in mL, both nurses and doctors made underestimations that were not significant.

Small to moderate amounts of blood in the toilet bowl are subjectively significantly overestimated, whereas larger volumes tend to be underestimated. This may have implications for the management of patients presenting with rectal bleeding.