MLT GlobeImmunology & Serology: Serologic tests & methods – page 3
76 Immunology & Serology MCQs on Serologic tests & methods with answers and explanations.
In the reverse syphilis screening algorithm, the treponemal EIA is reactive but the RPR is non-reactive. The next step is to:
A discordant EIA+/RPR− result needs a second treponemal test. If TP-PA is reactive it suggests past treated or untreated syphilis; if non-reactive the EIA is likely false-positive.
After successful treatment of early syphilis, which pattern is expected?
Non-treponemal titres fall at least fourfold after effective therapy and are used to follow treatment. Treponemal antibodies usually remain for life.
The ASO titre is most useful for diagnosing which condition?
ASO rises 1–3 weeks after group A streptococcal infection and supports diagnosis of rheumatic fever or glomerulonephritis. After skin infection ASO often stays low; anti-DNase B is better.
High-sensitivity CRP (hs-CRP) is mainly used to:
hs-CRP measures low CRP levels (below about 10 mg/L); values above 3 mg/L suggest higher cardiovascular risk. It should not be measured during acute illness.
A monospot (heterophile antibody) test is negative in a 3-year-old with clinical infectious mononucleosis. The most appropriate next test is:
Heterophile antibodies are often absent in young children and early in illness. EBV VCA IgM positive with EBNA negative indicates acute infection.
A patient has HBsAg negative, anti-HBc total positive and anti-HBs positive. This pattern most likely indicates:
Anti-HBc shows past natural infection, and anti-HBs with negative HBsAg shows recovery and immunity. Vaccination produces anti-HBs alone without anti-HBc.
The CH50 assay measures the reciprocal of the serum dilution that:
CH50 tests the whole classical pathway (C1–C9) using sheep red cells coated with rabbit antibody. Unsensitised rabbit red cells are used for the alternative pathway (AH50).
Paired sera for a viral infection show an acute-phase titre of 1:16 and a convalescent titre of 1:128 taken 2 weeks later and tested together. The best interpretation is:
A rise of four-fold or more (here eight-fold) between paired sera tested in the same run indicates recent infection. A one-dilution difference is within normal test variation.
In Ouchterlony double diffusion, the precipitin lines from two neighbouring antigen wells join, but one line extends past the junction as a spur. This indicates:
A spur means one antigen has all the epitopes of the other plus extra ones, so antibodies to the extra epitopes pass the junction: partial identity. Complete identity gives a smooth fused arc without a spur.
In the current CDC laboratory algorithm for HIV diagnosis, the initial screening test is:
The algorithm starts with a fourth-generation (or later) antigen/antibody immunoassay that detects p24 antigen and HIV-1/2 antibodies, shortening the window period. The Western blot is no longer part of the algorithm.
In a haemagglutination inhibition test for rubella antibody, a positive result is shown by:
Rubella virus agglutinates certain red cells. Patient antibody neutralises the virus so red cells do not agglutinate; the inhibition of agglutination is the positive result.
In a complement fixation test, the patient's serum contains the antibody being tested for. The indicator system will show:
Antigen-antibody complexes fix (use up) the added complement, so none is left to lyse the antibody-coated sheep red cells. Haemolysis means complement was free, i.e., no antibody.
The classical tube test for antistreptolysin O (ASO) antibody is based on:
Patient ASO neutralises streptolysin O so that it can no longer lyse rabbit or human red cells; the highest dilution without haemolysis gives the titre. Latex methods exist, but the classical test is neutralisation.
A 17-year-old with fever, sore throat and lymphadenopathy has EBV serology: VCA IgM positive, VCA IgG positive, EBNA IgG negative. This pattern indicates:
VCA IgM with VCA IgG and absent EBNA antibody fits acute primary infection; anti-EBNA appears only weeks to months later. Past infection shows VCA IgG and EBNA IgG without VCA IgM.
A TSH result by two-site sandwich immunoassay is unexpectedly high and falls to normal after the sample is treated with a heterophile blocking reagent. The interference was most likely due to:
Heterophile or human anti-mouse antibodies can link the capture and labelled antibodies without analyte, causing false-high sandwich results that correct with blocking reagents. Biotin causes falsely low sandwich results.
In the FTA-ABS test, the patient's serum is first mixed with a sorbent made from:
The sorbent from non-pathogenic Reiter treponemes removes cross-reacting group antibodies, improving specificity for T. pallidum. Cardiolipin antigen is used in non-treponemal tests such as RPR.
Which test on cerebrospinal fluid is most specific for diagnosing neurosyphilis?
A reactive CSF VDRL is highly specific for neurosyphilis (if the CSF is not blood-contaminated), although it lacks sensitivity. CSF FTA-ABS is sensitive and useful to exclude disease but less specific.
Latex particles coated with antibody are used to detect bacterial antigen in a sample. This technique is:
In reverse passive agglutination, carrier particles carry antibody and detect soluble antigen. Passive agglutination uses antigen-coated carriers to detect patient antibody.
Coagglutination tests use Staphylococcus aureus cells as carriers because of their:
Protein A binds IgG by its Fc portion, leaving Fab sites facing outward to agglutinate with antigen. Clumping factor binds fibrinogen, not antibody.
A jaundiced patient has HBsAg positive, HBeAg positive, IgM anti-HBc positive and anti-HBs negative. This indicates:
HBsAg with IgM anti-HBc indicates acute infection, and HBeAg shows active viral replication with high infectivity. Vaccination gives anti-HBs alone.