MLT GlobeImmunology & Serology: Serologic tests & methods – page 4
76 Immunology & Serology MCQs on Serologic tests & methods with answers and explanations.
A patient recovering from acute hepatitis B has HBsAg negative, anti-HBs negative and IgM anti-HBc positive. This stage is the:
In the window period HBsAg has cleared but anti-HBs is not yet detectable; IgM anti-HBc may be the only marker. Missing it would give a false-negative diagnosis.
A blood donor has a reactive anti-HCV immunoassay, but HCV RNA is not detected. The most likely interpretation is:
Antibody without detectable RNA suggests cleared past infection or a false-positive screen; a second antibody assay can separate these. Window-period infection would show RNA without antibody.
A traveller has had fever for 2 days. Which test is most useful to diagnose dengue at this stage?
NS1 antigen circulates from the first day of fever, usually for about 5 days, before IgM appears. IgG rises later, especially in primary infection, so it is not useful this early.
A patient has been HBsAg positive for more than 6 months. This indicates:
Persistence of HBsAg for longer than 6 months defines chronic hepatitis B infection. Vaccination produces anti-HBs only, and resolved infection shows anti-HBs with anti-HBc.
A test with very high sensitivity is most useful for:
A highly sensitive test misses few cases, so it gives few false negatives and is well suited to screening. High specificity is more important for confirmation.
A 2-month-old baby of an HIV-positive mother has a positive HIV antibody test. Why does this not confirm infection in the baby?
Maternal IgG crosses the placenta and can persist for up to 18 months, so infant diagnosis uses nucleic acid tests (HIV DNA or RNA) instead of antibody tests.
At her first antenatal visit, a pregnant woman is rubella IgG positive and rubella IgM negative. This result most likely means:
IgG without IgM indicates past infection or vaccination and immunity. Acute infection shows IgM, and a susceptible woman would be IgG negative.
In radial immunodiffusion read at endpoint (Mancini method), antigen concentration is proportional to:
At completion (Mancini), diameter squared is directly proportional to antigen concentration. In the timed Fahey method, diameter is proportional to the log of concentration.
A patient with post-streptococcal glomerulonephritis after impetigo has a normal ASO. Which antibody test is more likely to be elevated?
Streptolysin O is partly inactivated by skin lipids, so ASO response after skin infection is weak. Anti-DNase B rises after both pharyngeal and skin infections.
On a flow cytometry CD45 versus side-scatter plot, which population shows bright CD45 with low side scatter?
Lymphocytes are small, agranular and express the brightest CD45. Granulocytes have high side scatter and dimmer CD45; blasts show dim CD45 with low side scatter.
In a multicolour flow cytometry panel, the emission of FITC is partly detected in the PE channel. The correction applied for this is called:
Compensation mathematically subtracts spectral overlap of one fluorochrome into another detector. Gating selects cell populations for analysis but does not correct spillover.
A patient's HIV-1/2 antigen/antibody combination assay is reactive, but the HIV-1/HIV-2 antibody differentiation immunoassay is negative. The next step is:
A reactive screen with negative differentiation may be acute HIV-1 infection (antigen present before antibody) or a false-positive screen. An HIV-1 RNA test resolves this; reporting negative could miss acute infection.
In the differential absorption (Davidsohn) test, the heterophile antibody of infectious mononucleosis is:
The infectious mononucleosis heterophile antibody is not a Forssman antibody, so it is not removed by guinea pig kidney but is removed by beef red cells. Forssman antibodies are removed by guinea pig kidney.
A patient taking high-dose biotin has testing on streptavidin-biotin immunoassays. Which effect is expected?
Excess free biotin blocks streptavidin binding of the biotinylated complex. In sandwich assays signal falls (falsely low); in competitive assays low signal is read as high analyte (falsely high).
Which finding in a newborn best supports congenital syphilis?
Maternal IgG crosses the placenta, so a reactive infant IgG treponemal test or an equal RPR titre may be only passive antibody. An infant titre at least fourfold higher than the mother's suggests infant production.
In an indirect IgM assay for rubella, a false-positive IgM result can occur when the specimen contains:
IgM rheumatoid factor binds specific IgG attached to antigen and is detected by anti-IgM conjugate. Removing IgG before testing, or using IgM-capture formats, prevents this.