In connection with syphilis control, the standard test for measuring nontreponemal antibodies is the Venereal Disease Research Laboratory (VDRL) test. In this test, heated serum or unheated cerebrospinal fluid is mixed with reagin (a purified mixture of lipids such as cardiolipin, lecithin, and cholesterol) on a glass slide, and flocculation, or clumping, of the mixture is read microscopically as "reactive" (if clumping occurs) or "nonreactive" (if there is no clumping). Like the rapid plasma reagin (RPR) test, the VDRL test can be quantitated by examining serial dilutions of serum and can be used to follow the course of illness, including the response to therapy. The VDRL usually becomes reactive within the first few weeks after infection, peaks during the first year, and then slowly declines, so that low titers (levels) are seen in late syphilis. It can revert to negative in the absence of treatment in about 25 percent of cases. Although regarded as the gold standard for the diagnosis of neurosyphilis, the VDRL test may be negative in 40 to 73 percent of patients. Recent immunization, other bacterial and viral infections, and certain chronic conditions (e.g., liver disease, malignancy) can result in false positive test results, though titers are usually low (less than 1:8 dilution) under these circumstances.