"About a cupful"--a prospective study into accuracy of volume estimation by medical and nursing staff

In this prospective study, medical and nursing health care professionals were asked to visually estimate various fluid volumes. The aims were to assess overall accuracy, and to identify factors that caused error. The effects of fluid volume, colour and clinical experience were analysed. 152 doctors and nurses were shown a selection of containers filled with five different volumes of fluid, each presented in three different colours. Subjects recorded their estimates of volume by filling in a questionnaire. There was considerable variation in accuracy between subjects. Very large standard deviations were noted, and some subjects were found to over-report, to a maximum of 700%. Nurses were more accurate than doctors (p=0.0003). Nurses' accuracy was influenced by fluid colour but not volume, whereas the opposite was true for doctors. Doctors with more than 20 years' clinical experience significantly overestimated volumes compared to their younger colleagues. This effect was not seen with the nursing group. We conclude that visual estimation is unreliable, and recommend against using visual estimation in clinical practice.