Doctors can screen for infectious agents or diseases by looking for the presence of marker proteins. Immunological assays are often used for this, but they cannot detect a marker protein unless a certain amount of it is present in a blood sample. This means that sometimes the disease must already have progressed substantially to be detected. Researchers from the University of Pennsylvania in Philadelphia have devised a technique that analyzes low-abundance proteins in blood. As reported in the April issue of Nature Medicine, the researchers created a method they call fluorescent amplification catalyzed by T7 polymerase technique (FACTT). It uses RiboGreen, a fluorescent dye that detects RNA, to sense subfemtomolar levels of protein. Once a double-stranded DNA template has bound to the targeted infection-related protein molecules, RNA polymerase amplifies the molecules, and the resulting molecules are highlighted by RiboGreen. The researchers compared the detection of the Her2 breast cancer marker with ELISA (enzyme-linked immunosorbent assay) and FACTT in mouse blood. FACTT detected the cancer marker in three out of seven mice after two days and in all seven after 11 days. The ELISA technique could not detect any Her2, even after 11 days. They also compared the detection of Her2 in human blood between the two techniques. Using FACTT, nine out of 10 patients who tested positive for Her2 were identified, and using ELISA, only two showed elevated levels of Her2. The investigators believe that the results show promise for the use of the technology for monitoring infection and disease at early stages.