[Quantification of blood loss. How precise is visual estimation and what does its accuracy depend on?]

Abstract

Estimation of blood loss is a difficult task. Apart from measuring the volume of the suctioned blood the anaesthetist has to make a visual estimate of blood shed on the floor, spread in the surgeons' gowns and gloves and hidden in drapes and sponges at nearly every operation. We were interested in how exact visual estimation of blood loss can be and what factors influence accuracy and precision of the visual estimate. In one OR we simulated typical blood loss scenes occurring during a mock hip joint replacement, using our normal customary equipment of drapes, sponges and containers. More than 8 litres of blood from autologous whole blood donations were partitioned with a graduated measure and syringes and were distributed around the OR in 22 locations in typical ways. 36 members of staff entered the OR one by one and all gave their 22 estimates. Results were analysed by repeated measures analysis of variance. Bias (accuracy) and variation error (precision) were calculated for individuals and groups of individuals. We found a broad deviation of the visual estimates and little coincidence with the actual values. Over- and underestimations by 2 or even 3 were rather common; underestimations were more frequent. We found a significant trend to overestimate diluted blood, even though these certain sites were known to exhibit diluted blood. On the other hand laparotomy pads and sponges fully saturated with blood as well as the simulation of the operative site were grossly underestimated. Age, sex and professional experience (!) did not influence the magnitude of estimation errors, but the professional groups'estimates differed: anaesthetists estimated slightly but significantly more than orthopedic or general surgeons. Obviously our capability to estimate lost blood volumes is more influenced by our belonging to a professional group than by our professional experience. Do we not learn by experience? Diluted blood is overestimated, whereas in some other typical scenes blood loss is grossly underestimated. Simulations such as this one may improve our estimation capabilities and thus result in better patient care in the OR.