The purpose of this study is to measure the consumer understanding of medical services on blood tests. For this reason, the study joins the growing attention that marketing has given to the impact of functional illiteracy in consumer behavior, particularly in relation to the use of medical services. Interviews with physicians and research in sites and blogs related to medical information were used to develop the test. The elaboration of the test was inspired by two measuring instruments SILS (Single Item Literacy Screener The) and the TOFHLA (Test of Functional Health Literacy in Adults). The test was applied to a sample of patients who had blood drawn recently. The results allowed us to classify the respondents as either their understanding of medical instructions as the results of blood tests. This classification shows significant gaps in consumer understanding. The results also show the kind of attention that marketers who work in laboratories that perform blood tests and other establishments that provide medical services should give the problem.