Interpretive Handbook

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 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.

IgG is only indicative of previous exposure to toxoplasma (recent or past) and is present in up to 70% of the adult United States population. However, the absence of IgG is helpful in that it usually indicates the absence of infection.

Seroconversion from negative to positive IgG is indicative of recent Toxoplasma gondii infection. Seroconversion indicates infection subsequent to the first negative specimen.

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)

IgG is not useful for diagnosing infection in infants <6 months of age. IgG antibodies in that age group are usually the result of passive transfer from the mother.

Results should be interpreted with consideration of clinical and laboratory findings. A negative result does not necessarily indicate absence of infection, especially in immunosuppressed patients or very early in infection when antibody titers are low.

Toxoplasma ANTIBODY, IgG

<4 IU/mL (negative)

4-7 IU/mL (equivocal)

> or =8 IU/mL (positive)

Toxoplasma ANTIBODY, IgM

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

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