Every year, our laboratory can be re-certified in a laboratory performance test. To this end, it conducts proficiency tests to ensure that our test results remain reliable and meaningful for the analyses within the reference ranges. The certification is carried out by the INSTAND e.V. - Association for the Promotion of Quality Assurance in Medical Laboratories e.V. Regrettably, in Germany. There are no uniform measuring methods for some measurements. This allows any laboratory to freely decide which method to use. For example, there are several evaluation methods for determining the vitamin D level which have been acknowledged as reliable methods in various studies and hardly differ in their quality. Nevertheless, there are differences in the methods, such as the speed of the evaluation and the specified tolerance ranges. This leads, among other things, to the fact that every laboratory has a scatter in the measurement results. So it can unfortunately come to a very different measurement results. Consequently, only comparisons between identical test procedures and test conditions are meaningful.