Molecular diagnostics is the analysis of small molecules, typically genetic material and proteins expressed in an organism, that correlate with a pathogenesis and therefore may be used as a marker of disease. Often the molecular marker is a causative agent for the disease state, although the marker may also be indirectly linked to the pathogenesis.

Molecular markers emerged as an efficient, reliable and non-invasive mean of diagnosis in the late twentieth century, and are today established as the gold standard in the diagnosis of numerous diseases and conditions, including infectious diseases, cancer typing and hereditary diseases. As genetic material and most signaling proteins are readily available in blood, a blood test is very often sufficient as the sampling method.