Question

Given below are two statements:
Statement (I): Sabin-Feldman dye test is a sensitive and specific method for diagnosis of toxoplasmosis.
Statement (II): If the tachyzoites do not stain with 1% methylene blue stain, then the test is negative.
In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both Statement (I) and Statement (II) are true.
• B. Both Statement (I) and Statement (II) are false.
• C. Statement (I) is true but Statement (II) is false.
• D. Statement (I) is false but Statement (II) is true.

Hint:

Remember the unique principle of the Sabin-Feldman dye test: Antibodies kill/neutralize the tachyzoites, preventing them from staining. Thus, Unstained = Positive for antibodies; Stained Blue = Negative for antibodies.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The Sabin-Feldman dye test (SFDT) is the gold standard serological reference method used for the diagnosis of toxoplasmosis, caused by the protozoan Toxoplasma gondii.
The test is based on antibody-mediated complement-dependent neutralization of live Toxoplasma tachyzoites.

Step 2: Detailed Explanation:
Let us evaluate both statements:
- Statement (I): The Sabin-Feldman dye test is a highly sensitive and specific method for detecting antibodies against Toxoplasma gondii in serum. It has long been recognized as the reference serological test in diagnostic parasitology. Thus, Statement (I) is true.
- Statement (II): The underlying principle of the test is that live, normal tachyzoites readily take up alkaline 1% methylene blue stain and appear blue under a microscope. However, when live tachyzoites are incubated with patient serum containing specific anti-Toxoplasma antibodies along with accessory factor (complement), the antibodies bind to the cell membrane, activating the complement pathway. This activation lyses or neutralizes the tachyzoites, preventing them from taking up the methylene blue dye. As a result, in a positive test (where antibodies are present), the tachyzoites remain unstained and colorless. If there are no antibodies (a negative test), the tachyzoites are undamaged and stain blue. Thus, Statement (II) is incorrect because unstained tachyzoites indicate a positive test, not a negative one.
Therefore, Statement (I) is true but Statement (II) is false.

Step 3: Final Answer:
Statement (I) is true but Statement (II) is false, which corresponds to option (C).