Toxoplasma Antibody, IgM, Serum
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Toxoplasma gondii is an obligate intracellular parasite that is capable of infecting a variety of intermediate hosts including humans. Infected definitive hosts (cats) shed oocysts in feces that rapidly mature in soil and become infectious. Toxoplasmosis is acquired by humans via ingestion of food or water contaminated with cat feces or undercooked meats containing oocysts.
Infection of the normal adult is commonly asymptomatic. In cases with clinical manifestations, the most common symptom is lymphadenopathy, which may be accompanied by an array of other symptoms making differential diagnosis difficult.
Severe-to-fatal infections do occur in adults immunocompromised by cancer chemotherapy or other immunosuppressive treatment, and in patients with AIDS. These infections are thought to be caused by reactivation of latent infections and often involve the central nervous system.
Transplacental transmission of the parasites resulting in congenital toxoplasmosis can occur during the acute phase of acquired maternal infection. The risk of fetal infection is a function of the time at which acute maternal infection occurs during gestation. The incidence of congenital toxoplasmosis increases as pregnancy progresses; conversely, the severity of congenital toxoplasmosis is greatest when maternal infection is acquired early during pregnancy. A majority of infants infected in utero are asymptomatic at birth, particularly if maternal infection occurs during the third trimester, with sequelae appearing later in life. Congenital toxoplasmosis results in severe generalized or neurologic disease in about 20% to 30% of the infants infected in utero; approximately 10% exhibit ocular involvement only and the remainder are asymptomatic at birth. Subclinical infection may result in premature delivery and subsequent neurologic, intellectual, and audiologic defects.
Aids in the diagnosis of both congenital and acute acquired toxoplasmosis
Diagnosis of acute central nervous system, intrauterine, or congenital toxoplasmosis is difficult by routine serological methods. Active toxoplasmosis is suggested by the presence of IgM antibodies, but elevated anti-IgM titers are often absent in immunocompromised patients. In addition, elevated IgM can persist from an acute infection that may have occurred as long ago as 1 year.
A suspected diagnosis of acute toxoplasmosis should be confirmed by further testing at a toxoplasmosis reference laboratory, or by detection of Toxoplasma gondii DNA by PCR analysis of cerebrospinal fluid or amniotic fluid specimens (PTOX/81795 Toxoplasma gondii, Molecular Detection, PCR).
For confirmation of a diagnosis, the FDA issued a Public Health Advisory (7/25/1997) suggesting that sera found to be positive/equivocal for Toxoplasma gondii IgM antibody be sent to a Toxoplasma reference laboratory. CDC or Jack Remington, M.D., Palo Alto Medical Foundation, 860 Bryant Street, Palo Alto, CA 94301, were recommended. (Reviewed 12/2011)
Specimens interpreted as equivocal may contain very low levels of IgM. A second specimen should be drawn and tested.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Negative results do not preclude recent primary Toxoplasma gondii infection. A negative result could indicate either no previous exposure or also could be seen in cases of remote exposure with subsequent loss of detectable antibody. A second specimen drawn at a later point in time may be needed to rule out a recent infection.
Positive serologic results alone are not diagnostic of Toxoplasma gondii infection. For example, infections with Epstein-Barr virus (EBV) have been suspected to elicit antigen-specific IgM responses (eg, false-positive IgM Toxoplasma reactions) in individuals previously sensitized to a variety of non-EBV infectious agents.
Since persisting IgM levels may be detected long after the onset of acquired infection, the use of a single serological test result must be used with caution in those cases when it is critical to establish the time of infection. This applies to the diagnosis of acute Toxoplasma gondii infection acquired during pregnancy. Determination of the date of infection based solely on the results of detectable IgM antibody to Toxoplasma gondii is not recommended. That determination should include clinical history and previous serology, since low levels of IgM antibody may persist for a year or more. The use of a test to determine a rise in IgG antibody to Toxoplasma gondii (TOXOG/8267 Toxoplasma Antibody, IgG, Serum or TOXGM/81647 Toxoplasma Antibody IgG and IgM, Serum) may provide additional information as to the date of infection. Therefore, the FDA has instructed commercial suppliers of Toxoplasma IgM kits to recommend Toxoplasma IgG testing also be performed.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
Test value threshold <0.55 is negative
Test value threshold > or =0.55-<0.65 is equivocal
Test value threshold > or =0.65 is positive
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Luft BJ, Remington JS: Toxoplasmic encephalitis in AIDS. Clin Infect Dis 1992 August;15(2):211-222
2. Wong SY, Remington JS: Toxoplasmosis in pregnancy. Clin Infect Dis 1994 June;18(6):853-862