Hayward, Calif.-based Singulex, a developer of sensitive molecular-detection systems, raised $19 million in a fourth funding round. The company’s release is here.

The company is developing an analysis system, called Erenna, plus related “assays,” or specially designed biochemical tests, that are designed to detect individual protein molecules in biological samples such as blood. Such tests could be useful for detecting or diagnosing disease, monitoring the effect of drug treatment, and other research and diagnostic purposes. For instance, Singulex says its system can identify previously undetectable cellular levels of cardiac troponin I, a protein biomarker that can be used to measure tissue damage following a heart attack. The company claims that its system can actually count the number of proteins in a given sample.

Singulex already offers its services to research centers at biotech and pharmaceutical companies, and hopes also to break into the field of molecular diagnostics — a much more challenging and heavily regulated area.

The round was led by OrbiMed Advisors, joined by existing investors Fisk Ventures (a venture fund of H. Fisk Johnson, CEO of S.C. Johnson & Son), Prolog Ventures, and Advantage Capital. Singulex has raised $31.6 million to date, according to VentureWire (subscription required).