This paper outlines the methods and approaches used for the laboratory detection and investigation of protein C (PC) deficiency. It does not make recommendations as to which patients should have thrombophilia testing performed; this should be done in line with local guidance. Interpretation of PC level is complicated because level varies with age, and many conditions can cause acquired deficiency. Protein C is most usually measured by chromogenic assay as a part of the thrombophilia screen. There exists, however, a very small group of individuals with significant PC deficiency, in whom the chromogenic PC assay is normal. The coagulometric assay of PC is more sensitive to these rare defects, but these assays may lack specificity. Genetic analysis allows definitive diagnosis and may be useful in confirming that deficiency is inherited and not acquired and is particularly valuable in families with severe PC deficiency.