The serodiagnosis of Toxoplasma gondii infection is widely used for screening pregnant women in order to prevent congenital toxoplasmosis. The Sabin-Feldman dye test was the first test system able to detect and quantitate specific antibodies to Toxoplasma gondii at low levels and to differentiate acute and latent infection. The test is based on complement-mediated cytolysis of antibody-coated live T.gondii tachyzoites, which is indicated by their inability to take up methylene blue stain.
Toxoplasma gondii is an obligate intracellular protozoal parasite of many vertebrates including humans. If a mother is infected with Toxoplasma during pregnancy, it can cross the placenta and cause congenital anomalies in the fetus but yet remain unrecognized in women. Congenital toxoplasmosis can cause miscarriage or stillbirth. In many cases infants appear healthy at birth but may develop untoward sequelae of the infection later in life, including decreased vision or blindness, decreased hearing or deafness, and mental and psychomotor retardation.
The Sabin-Feldman dye test is a sensitive and specific neutralization test for toxoplasmosis and is regarded as the gold standard for detecting Toxoplasma antibodies in humans. This test is not performed in most laboratories because live virulent strains of T.gondii are needed for the test.
Sabin-Feldman dye test is used primarily as a confirmatory test in reference laboratories.
When live virulent tachyzoites of Toxoplasma gondii are incubated with normal serum, tachyzoites become swollen and stain deeply blue when methylene blue is added.
Tachyzoites when incubated in serum-containing specific antibodies to Toxoplasma gondii, under the same conditions, appear thin and distorted and are not stained when the dye is added. This is due to the activation of the complement system and lysis of the organisms. Dilution of serum at which half of the organisms are not killed (stained) and the other half are killed (unstained) is reported as the antibody titre.
Differentiation of lysed from nonlysed organisms may be readily accomplished under phase contrast microscopy, in that case, staining is not required.
Limitations of Sabin-Feldman dye test
- Highly Expensive, laborious and time-consuming: Live tachyzoites of Toxoplasma gondii is maintained by inoculating in the peritoneal cavity of mice.
- Requirement of an experienced technician
- Dangerous to laboratory workers because they may acquire toxoplasmosis while handling live virulent T. gondii.
- Use of animal may be undesirable from an ethical viewpoint
- Limited usefulness as it does not differentiate between current and past infection.
- Sabin AB, Feldman HA (1948). Dyes as microchemical indicators of a new immunity phenomenon affecting a protozoan parasite (Toxoplasma) Science 108, 660-3.
- Reiter-Owana I, Petersen E, Joynson D et al (1999). The past and present role of the Sabin-Feldman dye test in the serodiagnosis of toxoplasmosis. Bull World Health Organisation, 77(11), 929-935.