Some years ago, a new class of enzymes, the quinoproteins, were described. They are oxygen-independent oxidases. Common to this class of enzymes is the content of pyrroloquinoline quinone, which acts as a redox centre. These enzymes can transfer electrons from substrates such as glucose to electron acceptors such as cytochromes. They do not transfer electrons to oxygen. This property is very interesting with respect to the use of these enzymes in clinical chemistry, especially in assays where oxygen may have an influence on the results. Several examples of newly designed tests including biosensors for glucose and cholesterol will be discussed. The application of this type of enzyme has led to diagnostic tests that are superior in performance and reliability of test results.