In many cases, direct detection methods are not possible, are difficult and expensive, are unreliable, or are attempted with negative results. Serologic tests attempt to detect antibodies formed against antigens of an organism being searched for. The majority of organisms have a reasonably predictable antibody response. IgM-type antibodies appear first, most often in 7-14 days (sometimes later), generally reaching peak titer about 2 weeks later, then falling, and becoming nondetectable about 6 months later (usual range, 4-8 months, although sometimes longer). IgG-type antibodies typically appear about 2 weeks after initial appearance of IgM antibodies, peak about 2 weeks later, and typically persist for years (although some eventually disappear and others may slowly decrease in titer and persist at low titers). If antibodies to a particular organism are rare in a community, a single significantly elevated result can be at least presumptively diagnostic. However, most often it requires two serum specimens, one obtained immediately and the other 2-3 weeks later, hoping to obtain a fourfold (2-tube serial dilution) rise in titer in order to prove current or recent infection. Potential problems include circumstances in which serologic tests for certain organisms are not available, tests ordered to detect one organism when another is the culprit, patients who fail to produce detectable antibody, patient antibody formation that varies in time from the usual pattern, serum specimens obtained before antibody rise or after antibody becomes nondetectable, and specimens obtained after antibody has peaked so that past infection cannot be differentiated from current infection. In the last of these circumstances, presence of IgM antibody in high titer would suggest current or relatively recent infection. Finally, there is substantial variation in homemade or commercially available test kits, both in ability to detect the desired antibodies (sensitivity) and the number of false positive results obtained (specificity).