The aim of this work is to study the appropriateness of the laboratory measurement for quantities that may be conditioned by the result of another quantity, estimating the proportions of inappropriate requests due to over- or under-utilisation. We refer to over-utilisation when a quantity was requested and measured when it was not needed, and we refer to under-utilisation when a quantity was not requested and not measured when it was needed. In our centre, the laboratory staff together with the primary health care physicians of the region designed a new laboratory request form. This request form incorporated an agreed requesting protocol for the most common diagnoses in primary health care. Protocols included, according to each case, one or several of 11 pairs of quantities depending on each case. Second quantity in each pair was measured or not, conditional on the result of the first quantity of the pair, the age, the sex and the diagnosis corresponding to the requested protocol. Following recommendations from scientific societies, we decided what should be the results of the first quantity in the pair so to measure the second quantity in the pair. When recommendations were not available, we reviewed the data contained in the laboratory information system, and we established as non-conditioning values, the levels of the first quantities that were associated with the results of second quantities within their reference intervals in 99.95% of cases. The study was done retrospectively using 46091 old laboratory request forms received in our laboratory over two years. Among the 46091 request forms, the first quantities included in the present forms that incorporate agreed requesting protocols were requested 66434 times. The 66434 corresponding second quantities were measured and should not have been measured in 14225 cases (21.4%) and were not measured and should have been measured in 16137 cases (24.3%). Using the request forms according to the conditioning protocols changes the number of quantities by a mean of 0.66 in every request form. This change would have increased the number of quantities measured by 0.04 quantities per form, but would also have improved the appropriate use of the laboratory.