In an agglutination test, eight serial dilutions to determine antibody titer were set up: Tube 1 contained a 1:2 dilution; tube 2, a 1:4, and so on. If tube 5 is the last tube showing agglutination, what is the antibody titer?
a. 5
b. 1:5
c. 32
d. 1:32

Verified step by step guidance

Understand that in an agglutination test, the antibody titer is defined as the highest dilution of serum that still shows visible agglutination.

Identify the dilution factor for each tube. Given that tube 1 is a 1:2 dilution, tube 2 is 1:4, and the dilution doubles with each subsequent tube, the dilution for tube n can be expressed as $1:\left(2^n\right)$.

Calculate the dilution for tube 5 by substituting n = 5 into the formula: $1:\left(2^5\right)$.

Recognize that the antibody titer corresponds to the reciprocal of the highest dilution showing agglutination, which is the dilution factor of tube 5.

Express the antibody titer as the reciprocal dilution, which will be in the form \$1:32$ for tube 5.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Serial Dilution in Agglutination Tests

Serial dilution involves stepwise dilution of a sample, usually by a constant factor, to reduce concentration systematically. In agglutination tests, each tube contains a progressively diluted antibody solution, allowing determination of the highest dilution that still causes visible clumping (agglutination). This helps quantify antibody concentration.

Antibody Titer Definition

Antibody titer is the highest dilution of serum that still produces a positive reaction, such as agglutination. It is expressed as the reciprocal of the dilution factor (e.g., 1:32 means the serum is diluted 32 times). The titer indicates the strength or concentration of antibodies in the sample.

Interpreting Agglutination Test Results

In agglutination tests, the last tube showing visible clumping indicates the antibody titer. For example, if tube 5 with a 1:32 dilution is the last positive, the antibody titer is 32 (or 1:32). This value helps assess immune response or infection status.

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