MLT GlobeImmunology & Serology: Serologic tests & methods – page 2
76 Immunology & Serology MCQs on Serologic tests & methods with answers and explanations.
The Widal test is used in the diagnosis of:
The Widal test detects agglutinating antibodies to Salmonella Typhi and Paratyphi O and H antigens in enteric fever.
In acute hepatitis B infection, which serological marker is usually the first to appear?
HBsAg appears in blood first, often before symptoms. Anti-HBs appears late, after recovery or vaccination.
Which enzyme is commonly used as the label in ELISA tests?
Horseradish peroxidase and alkaline phosphatase are the most common ELISA labels; they convert a colourless substrate into a coloured product.
Which specimen is most commonly used for antibody (serological) testing?
Most serological tests are performed on serum from clotted blood. Other fluids are used only for specific tests.
Why are positive and negative controls included with each run of a serological test?
Controls with known reactivity show that reagents and procedure are working. If controls fail, patient results cannot be reported.
In a latex slide agglutination test for C-reactive protein, visible clumping indicates that:
The latex particles are coated with anti-CRP antibody. CRP in the serum cross-links the particles, producing visible agglutination, which is read as positive.
In HIV testing, the 'window period' is the time between:
During the window period the person is infected (and can transmit infection) but antibodies or antigen are not yet detectable, so a test may be falsely negative.
In a sandwich ELISA for an antigen, the colour produced is:
More antigen captures more enzyme-labelled detection antibody, so more colour develops. In competitive immunoassays the signal is inversely related to analyte.
Which of these laboratory tests is based on an agglutination reaction?
In the RF latex test, IgG-coated latex particles clump visibly when RF binds them. Immunodiffusion and immunofixation are precipitation methods; ELISA uses enzyme labels.
The Paul–Bunnell test for infectious mononucleosis detects heterophile antibodies by:
EBV-induced heterophile IgM antibodies agglutinate sheep (or horse) erythrocytes. The Davidsohn absorption step improves specificity.
Latex beads coated with a soluble antigen clump when mixed with patient serum containing the matching antibody. This format is called:
Coating a soluble antigen onto inert carriers (latex, red cells) turns a precipitation reaction into visible agglutination: passive agglutination. Coagglutination uses antibody bound to staphylococcal protein A.
Coagglutination reagents use killed bacteria whose protein A binds the Fc portion of IgG. Which organism is used?
S. aureus Cowan I is rich in protein A, which binds IgG Fc and leaves the Fab arms free to agglutinate with antigen.
Red cells coated with IgG antibodies that cannot bridge cells on their own are made to agglutinate by adding anti-human globulin. Which technique is this?
Incomplete (non-agglutinating) IgG sensitises red cells without visible clumping; anti-human globulin cross-links the bound IgG, producing agglutination.
Many IgG anti-Rh antibodies coat red cells in saline but fail to produce visible clumping. Such antibodies are termed:
IgG molecules are too small to span the distance between red cells in saline, so they sensitise without agglutinating; they are detected with antiglobulin or enhancement media.
A strongly positive serum gives a negative agglutination result when tested undiluted but positive results at higher dilutions. This is:
When antibody far exceeds antigen, each antigen site is covered by separate antibodies and lattices cannot form, giving a false negative. Testing serial dilutions reveals the true positive.
A serum is tested in doubling dilutions starting at 1:2. Agglutination is seen in tubes 1 to 6 but not in tube 7. The titre is:
Tube 1 = 1:2, tube 2 = 1:4 ... tube 6 = 1:64. The titre is the reciprocal of the highest dilution with a positive reaction, so 64.
In Ouchterlony double diffusion, precipitin lines that cross each other completely indicate:
Crossing lines mean the two antigens share no epitopes (non-identity). Fused arcs indicate identity and a spur indicates partial identity.
Immunofixation electrophoresis differs from protein electrophoresis because it:
After electrophoresis, specific antisera (anti-G, A, M, kappa, lambda) are applied to precipitate and type a monoclonal protein. It is more sensitive than serum protein electrophoresis for small bands.
In a competitive immunoassay, a higher concentration of analyte in the patient sample produces:
Patient analyte competes with labelled analyte for limited antibody. More patient analyte means less labelled analyte bound, so signal is inversely proportional to concentration.
A one-step sandwich immunoassay gives a falsely low result for an extremely high hCG level. The best troubleshooting step is to:
Very high antigen saturates both capture and detection antibodies (high-dose hook effect). Diluting the sample brings it into the measuring range and reveals the true high value.