IgM testing is especially helpful for diseases that have nonspecific clinical presentations, such as toxoplasmosis, and for conditions that require rapid therapeutic decisions. For example, rubella infection in pregnant women can lead to congenital defects in the unborn fetus, such as cataracts, glaucoma, mental retardation, and deafness. Therefore, pregnant women who are exposed to rubella virus and develop a mild febrile illness can be tested for the presence of anti-rubella IgM. In addition, identification of IgM within the amniotic fluid of a pregnant mother is diagnostic of neonatal infection. Because IgG can readily cross the placenta, newborns carry titers of IgG passed from the mother to the fetus during the first 2 to 3 months of life until the infant produces his or her own antibodies. This is the only form of natural passive immunity. Accurate serologic diagnosis of infection in neonates requires either demonstration of a rise in titer (which takes time to occur) or the detection of specific IgM directed against the putative agent. Because the IgM molecule does not cross the placental barrier, any IgM would have to be of fetal origin and diagnostic of neonatal infection. Agents difficult to culture or those that adult females would be expected to have encountered during their lifetimes, such as Treponema pallidum, cytomegalovirus, herpes virus, Toxoplasma sp., or rubella virus, are organ isms that may cause an infection and elevation of fetal IgM. The names of some of these agents have been grouped together with the acronym STORCH (syphilis, Toxoplasma sp., rubella, cytomegalovirus, and herpes). These tests should be ordered separately, depending on the clinical illness of a newborn suspected of having one of these diseases. In many instances, however, infected babies display no clinical signs or symptoms of infection. Furthermore, in many cases serologic tests yield false-positive or false-negative results. Therefore, multiple considerations, including the patient history and the clinical signs and symptoms, must be included in the serodiagnosis of neonatal infection, and in many cases culture is still the most reliable diagnostic method.

Several methods have been developed to measure specific IgM in sera that may also contain IgG. In addition to using a labeled antibody specific for IgM as the marker or the IgM capture sandwich assays, the immunoglobulins can be separated from each other by physical means. Centrifugation through a sucrose gradient, performed at very high speeds, has been used in the past to separate IgM, which has a greater molecular weight than IgG.

Other available IgM separation systems use the presence of certain proteins on the surface of staphylococci (protein A) and streptococci (protein G expressed by group C and G streptococci) that bind the Fc portion of IgG. A simple centrifugation step separates the particles and their bound immunoglobulins from the remaining mixture, which contains the bulk of the IgM. Other methods use antibodies to remove IgM from sera containing both IgG and IgM. An added bonus of IgM separation systems is that rheumatoid factor, IgM antibodies produced by some patients against their own IgG, often binds to the IgG molecules being removed from the serum. Consequently, these IgM antibodies are removed along with the IgG. Rheumatoid factor can cause nonspecific reactions and interfere with the results in a variety of serologic tests.

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مواضيع ذات صلة

Complement Fixation Assays

Flocculation Tests

Particle Agglutination Tests

Direct Whole Pathogen Agglutination Assays

Serodiagnosis of Infectious Diseases

TSH- Receptor Antibodies

Thyroglobulin and Thyroid Peroxidase Antibodies

Interpretation of Serologic Tests

Features of the Humoral Immune Response Useful in Diagnostic Testing

Enzyme Immunoassays

Immunofluorescent assays

Particle Agglutination