The sensitivity of a certain technique is generally connected with the amount of microorganisms present in the specimen. The specificity of the technique used relies on the uniqueness of the morphology of the microorganism under the microscope or on how specific the antigen or the genetic probe that is used for that certain genus or species. For instance, the sensitivity of Cram strains is very high in which the observation of a Gram-stained smear of uncentrifuged urine containing two bacteria per oil immersion field (X1,000) is equivalent to the presence of > 105 colony forming units per ml of urine. In addition, the sensitivity of the Gram-stained smear for detecting the presence of Gram-negative coccobacilli form a specimen collected by cerebrospinal fluid of children that have been afflicted with Haemophilus influenzae meningitis is more or less 75 percent due to the fact that in some patients, the number of CFU per ml of cerebrospinal fluid is less than 104. And at least 104 colony forming units of the microorganism, tubercle bacilli per ml of sputum must be collected to be identified by an acid-fast smear of both decontaminated and concentrated sputum.
When the sensitivity of a test is increased there is a tendency of it to have a decrease in its specificity. For instance, the examination of a Gram-stained smear of sputum coming from a patient that is suffering form pneumonococcal pneumonia is highly sensitive; however, it is also highly nonspecific if the criterion for describing a positive test is the existence of any Gram-positive cocci. If, on the other hand, a positive test an be described by the existence of a prevalence of Gram-positive, lancet-shaped diplococci, the test is converted to a test that has high specificity, but nonetheless, the sensitivity of the given test drops down to about 50 percent.
The same problems about the sensitivity of immunoassays and genetic probes for fungi, chlamydiae, viruses and fungi are still affected by the number of microorganisms existing on the specimen. In several cases, the sensitivity of the direct examination tests can be enhanced by taking better samples to be examined. For instance, in testing for Chlamydia trachomatis, the fluorescent antibody stain has greater sensitivity when the cells obtained are from the endocervical region and was taken by though the use of cytobrush as compared to a swab. The sensitivity of the test can also be affected by the specimen collected at a certain stage of the disease. For instance, the identification of the herpes simplex virus by different tests such as immunoassay, immunofluorescence, or by the use of cultures is greatest when the samples came from lesions in the vesicular stage of infection are examined. Furthermore, the sensitivity of a given test can be enhanced by using an enhancement or enrichment step in which the microbial or genetic replication happens to the point at which the identification techniques can be applied.
