Valab® is an expert system used to assist the biological validation of medical test results. An expert system reproduces the reasoning of a human expert, reasoning which is based on choices taking into account the largest possible number of parameters. When a decision is based on one or two criteria only, the reasoning is relatively easy to master. To evaluate the coherence and/or the acceptability of a medical test result however, multiple criteria come into play: the age and sex of the patient, his/her state of health, if he/she is treated in a hospital or at home, the previous results obtained for the same test, etc. The reasoning rapidly becomes extremely complex. The assessment of the coherence / acceptability of the result must also take into account the results of all the other tests in the report. A classic algorithmic system used to assess the coherence / acceptability of a result will quickly be overwhelmed by so many combinatorial choices, only an expert system like Valab can handle such complexity.