Question:medium
A patient was suspected of having brucellosis. Serum sample was sent for a standard agglutination test. It came negative initially but after dilution of the serum sample, the test was positive. What could be the reason for the initial negative test?
A patient was suspected of having brucellosis. Serum sample was sent for a standard agglutination test. It came negative initially but after dilution of the serum sample, the test was positive. What could be the reason for the initial negative test?
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When an agglutination test is negative at high antibody concentration but positive after dilution, recall which zone of the antigen-antibody reaction is responsible.
Updated On: Jun 23, 2026
- Prozone
- Postzone
- Complement inactivation
- Antigen antibody complexes
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The Correct Option is A
Solution and Explanation
Immunology concept: Zones of antigen-antibody reaction
In an agglutination reaction, the precipitation/agglutination pattern depends on the ratio of antigen (Ag) to antibody (Ab):
| Zone | Ratio | Result |
|------|-------|--------|
| Prozone | Excess Ab | No agglutination (false negative) |
| Equivalence | Optimal Ag:Ab | Maximum agglutination |
| Postzone | Excess Ag | No agglutination (false negative) |
In this case: undiluted serum = high Ab concentration = prozone = false negative. On dilution, Ab concentration falls to the equivalence zone = agglutination seen = positive test.
Standard Agglutination Test (SAT) for brucellosis uses a titre of 1:160 or more as significant. False negative SAT occurs in:
- Prozone phenomenon
- Presence of blocking (incomplete IgG4) antibodies
\[\boxed{\text{Prozone}}\]
In an agglutination reaction, the precipitation/agglutination pattern depends on the ratio of antigen (Ag) to antibody (Ab):
| Zone | Ratio | Result |
|------|-------|--------|
| Prozone | Excess Ab | No agglutination (false negative) |
| Equivalence | Optimal Ag:Ab | Maximum agglutination |
| Postzone | Excess Ag | No agglutination (false negative) |
In this case: undiluted serum = high Ab concentration = prozone = false negative. On dilution, Ab concentration falls to the equivalence zone = agglutination seen = positive test.
Standard Agglutination Test (SAT) for brucellosis uses a titre of 1:160 or more as significant. False negative SAT occurs in:
- Prozone phenomenon
- Presence of blocking (incomplete IgG4) antibodies
\[\boxed{\text{Prozone}}\]
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