Toxoplasmosis Workup

Results from basic laboratory studies such as complete blood cell count (CBC), chemistries, and liver function tests (LFTs) are typically normal, although lymphocytosis may be present.

Direct detection

The diagnosis of toxoplasmosis is confirmed with the demonstration of T gondii organisms in blood, body fluids, or tissue. T gondii may be isolated from the blood via either inoculation of human cell lines or mouse inoculation. Mouse inoculation may require a longer time to yield results and also is likely to be more expensive. Isolation of T gondii from amniotic fluid is diagnostic of congenital infection by mouse inoculation.

Polymerase chain reaction assay testing on body fluids, including CSF, amniotic fluid, bronchoalveolar lavage fluid, and blood, may be useful in the diagnosis. However, PCR assay is capable of detecting T gondii deoxyribonucleic acid (DNA) in either an aqueous sample or a vitreous sample in only one third of patients with ocular toxoplasmosis.

Indirect detection

Indirect detection is performed in pregnant women and in immunocompromised patients. Detection of immunoglobulin G (IgG) is possible within 2 weeks of infection using the enzyme-linked immunosorbent assay (ELISA) test, the IgG avidity test, and the agglutination and differential agglutination tests. (Acute and convalescent sera have no role in the indirect detection of toxoplasmosis.)

Procedures

The following diagnostic procedures may be performed for toxoplasmosis:

• Lumbar puncture - After imaging to identify evidence of increased intracranial pressure
• Brain biopsy
• Lymph node biopsy
• Amniocentesis - Perform amniocentesis at 20-24 weeks' gestation if congenital disease is suggested
• Bronchoalveolar lavage

Tachyzoites may be demonstrated in tissues or smears obtained from biopsy. They also can be seen in CSF. CSF also shows mononuclear pleocytosis and elevated protein level. Tachyzoites demonstrate acute infection, while tissue cysts and bradyzoites are seen in chronic/latent infection (although they may be present in acute infection/reactivation).

Skin tests

Skin tests that show delayed skin hypersensitivity to T gondii antigens may be useful as a screening test.

Testing in pregnancy

Although testing in pregnancy may not be indicated and treatment may not have established literature support, a low index of suspicion is needed to identify acute infection in pregnant patients.

Ophthalmic disease

Antibody titers do not correlate with ophthalmic disease. Antitoxoplasmic antibodies may be very low and should be tested in undiluted (1:1) samples if possible. The absence of antibodies rules out the disease; nevertheless, false-negative results do occur.

Invasive techniques are usually reserved for difficult cases, such as patients who are immunocompromised. Ocular fluids can demonstrate the presence of intraocular antibody production. Polymerase chain reaction assay can detect the causative organism.

Acute systemic toxoplasmosis has traditionally been diagnosed by seroconversion. Anti-Toxoplasma immunoglobulin G (IgG) titers present a 4-fold increase that peak 6-8 weeks following infection and then decline over the next 2 years, although they remain detectable for life. Anti-Toxoplasma IgM appears in the first week of the infection and then declines in the next few months. The presence of anti-Toxoplasma IgA has also been shown to be detectable in acute infection; however, since the titers can last for more than 1 year, its value in helping to diagnose an acute phase is limited.

Detection of IgG is possible within 2 weeks of infection using the ELISA test, the IgG avidity test, and the agglutination and differential agglutination tests. The presence of IgG indicates a likely past infection, while the presence of IgM usually indicates acute infection (particularly in the absence of IgG). However, IgM has, in some cases, been documented to persist for months or years.

Lack of IgG and IgM may exclude infection. IgM alone that then transitions to IgG without IgM or both IgG and IgM indicates likely acute infection. There is a significant rate of false IgM positivity. The sensitivities and specificities of the commercially available IgM and IgG tests vary substantially.

Sabin-Feldman dye test

The Sabin-Feldman dye test is a sensitive and specific neutralization test for toxoplasmosis. It is used to measure primarily IgG antibody and is the standard reference test for toxoplasmosis. However, it requires live T gondii organisms; therefore, it is not available in most laboratories. (It is used primarily as a confirmatory test in reference laboratories.) High titers suggest acute toxoplasmosis.^[36]

Fluorescent antibody test

The indirect fluorescent antibody test is used to measure the same antibodies as the dye test. Titers parallel dye test titers. The IgM fluorescent antibody test is used to detect IgM antibodies within the first week of infection, but titers fall within a few months.

Hemagglutination test

The indirect hemagglutination test is easy to perform. However, it usually does not detect antibodies during the acute phase of toxoplasmosis. Titers tend to be higher and remain elevated longer.

ELISA test

The results from a double-sandwich IgM ELISA are more sensitive and specific than the results from other IgM tests.

IgG avidity test

The results of the IgG avidity test may help to differentiate patients with acute infection from those with chronic infection better than do alternative assays, such as assays that measure IgM antibodies. As is true for IgM antibody tests, the avidity test is most useful when performed early in gestation.

IgG produced early in infection is less avid and binds to T gondii antigens more weakly than do antibodies produced later in the course of infection. High antibody avidity indicates an older, earlier infection. This test may be helpful in the setting of pregnancy, as the timing of infection has prognostic value. A long-term pattern occurring late in pregnancy does not exclude the possibility that the acute infection may have occurred during the first months of gestation.^[37]

Head CT scanning in cerebral toxoplasmosis (general)

In most immunodeficient patients with toxoplasmic encephalitis, CT scans show multiple bilateral cerebral lesions. However, although multiple lesions are more common in persons with toxoplasmosis, they may be solitary. Therefore, a single lesion should not exclude toxoplasmic encephalitis as a diagnostic possibility.

Head CT scanning in cerebral toxoplasmosis (in patients with AIDS)

CT scans in patients with AIDS who have toxoplasmic encephalitis reveal multiple ring-enhancing lesions in 70-80% of cases. In patients with AIDS who have detectable Toxoplasma IgG and multiple ring-enhancing lesions on CT scans or MRIs, the predictive value for toxoplasmic encephalitis is approximately 80%.

Lesions tend to occur at the corticomedullary junction (frequently involving the basal ganglia) and are characteristically hypodense.

The number of lesions is frequently underestimated when assessed using CT scan images, although delayed imaging after a double dose of intravenous (IV) contrast material may improve the sensitivity of this modality. An enlarging, hypodense lesion that does not enhance is a poor prognostic sign.

Single-photon computed tomography

Single-photon computed tomography (SPECT) scanning is useful in distinguishing between CNS lymphoma and infection (ie, toxoplasmosis or any other infection).

PET scanning, radionuclide scanning, and MR techniques

Various positron emission tomography (PET) scanning, radionuclide scanning, and magnetic resonance techniques have been used to evaluate patients with AIDS who have focal CNS lesions and to specifically differentiate between toxoplasmosis and primary CNS lymphoma.

MRI

MRI has superior sensitivity (particularly if gadolinium is used for contrast) to CT scanning, and MRI scans often demonstrate a single or multiple lesion(s) or more extensive disease not apparent on CT scans. One study showed that MRI detected abnormalities in 40% of patients whose abnormalities were not detected on CT.^[38]

Toxoplasmic encephalitis lesions on MRIs appear as high-signal abnormalities on T2-weighted studies and have a rim of enhancement surrounding the edema on T1-weighted, contrast-enhanced images.

Hence, MRI should be used as the initial procedure when feasible (and especially if a single lesion is demonstrated on CT scan images). Nevertheless, even characteristic lesions on CT scans or MRIs are not pathognomonic of toxoplasmic encephalitis.

The major differential diagnosis of focal CNS lesions in patients with AIDS is CNS lymphoma, which manifests as multiple enhancing lesions in 40% of cases.

The probability of toxoplasmic encephalitis falls and the probability of lymphoma rises in the presence of single lesions on MRI scans. Therefore, a brain biopsy may be required to obtain a definitive diagnosis in patients with a solitary lesion (especially if confirmed with MRI).

CT scanning abnormalities improve after 2-3 weeks of treatment in approximately 90% of patients with AIDS who have toxoplasmic encephalitis. Complete resolution takes 6 weeks to 6 months; peripheral lesions resolve more rapidly than do deeper ones.

Smaller lesions usually resolve completely within 3-5 weeks as shown on MRI, but lesions with a mass effect tend to resolve more slowly and leave a small residual lesion.

A radiologic response to therapy lags behind the clinical response, with better correlation by the end of acute therapy.

Ultrasonography

Ultrasonographic diagnosis of congenital toxoplasmosis in a fetus is available at 20-24 weeks' gestation. Fetal or neonatal ultrasonography can be useful in cases of known or suspected maternal acute infection and transplacental infection. Findings are generally nonspecific but include ventriculomegaly and CNS calcifications, particularly in the basal ganglia.

Histopathologic data for human toxoplasmosis has been obtained mostly from autopsy studies in infants and immunodeficient patients with serious infections. Such knowledge in immunocompetent patients is limited.

Pathologic findings are usually obtained from lymph node biopsy specimens in these patients. Multiple brain abscesses are commonly found, often involving the cerebral cortex and deep gray nuclei, less often the brainstem and cerebellum, and rarely the spinal cord. (See the images below.)

In acute toxoplasmosis, lesions are composed of central necrotic foci with varying petechiae rounded by acute and chronic inflammation, vascular proliferation, and macrophage infiltration. Tachyzoites and bradyzoites in tissue cysts may be detected at the periphery of the necrotic foci. T gondii are commonly found on hematoxylin and eosin or Giemsa stains. However, parasites can be more easily described via immunohistochemical staining. The blood vessels in the area of necrotic lesions may demonstrate distinguished intimal proliferation or frank vasculitis with thrombosis and fibrinoid necrosis.

Chronic lesions are composed of small cystic fields containing a number of lipid- and hemosiderin-laden macrophages with surrounding gliosis. Parasites are difficult to find in older lesions.