9- Diagnosis of Viral Infections (Antigen Detection)

Question 1

what is antigen detection in the context of viral diagnosis?

Answer 1

the direct identification of viral antigens or proteins present in patient specimens - viral proteins can be structural components or secreted proteins

Question 2

list the ideal types of patients specimens for antigen detections, and what they are used to detect

Answer 2

nasopharyngeal aspirates, blood (plasma or serum), faeces and vesicle fluid

nasopharyngeal aspirates = detect cell-associated virus antigens, collected by inserting a tube down the back of the throat to obtain cells near the nasopharynx
- e.g. RSV, influenza

blood = detects free antigens or whole virus particles circulating in the bloodstream
- hep B, dengue virus

vesicle fluid = detects whole virus particles from blister fluid
- e.g. HSV, VZV

faeces = detects whole virus particles especially in infections that display GI symptoms
- e.g. rotavirus, adenovirus

Question 3

what are the advantages of antigen detection?

Answer 3

rapid

relatively inexpensive compared to NAATs

provides quick diagnosis especially in resource-limited areas

Question 4

what are the limitations of antigen detection?

Answer 4

less sensitive compared to molecular tests like PCR

limited to detecting specific viral proteins targeted by the assay

lower specificity due to potential cross-reactivity with related antigens

Question 5

list the three main methods for antigen detection, and what they detect

Answer 5

direct immunofluorescence, enzyme immunoassay, immunochromatographic methods

direct immunofluorescence - detects cell-associated viral antigens

enzyme immunoassay - detects free soluble antigens or whole virus particles

immunochromatographic methods - rapid, point-of-care tests (lateral flow tests) that detect viral antigens

Question 6

describe antigen detection by direct immunofluorescence

Answer 6

nasopharyngeal aspirate specimen collected by inserting a tube through the nose, collecting epithelial cells at the posterior pharynx

specimen is spread on microscope slide, left to fix and dry

fixed specimen is treated with fluorochrome-tagged antibodies that will bind to their specific suspected viral antigen

slides then viewed under UV light - if the antigen is present in the sample, its tagged antibody will bind and produce a detectable fluorescence

Question 7

what are the limitations of antigen detection by direct immunofluorescence

Answer 7

not as sensitive as other assays, potential false positives

time consuming preparing the slides and assay

requires specialised equipment – a microscope with UV abilities

less expensive than other methods, but still requires reagents and equipment

Question 8

what are immunochromatographic tests? what are their pros and cons:

Answer 8

immunochromatographic test = lateral flow test; relies on the specific binding of antibodies to target antigens to produce a visible colour change

pros - detects antigens in a rapid, point-of-care method, no need for complex tests, quick diagnosis allows for quick intervention

cons - not as sensitive as NAATs/ PCR

Question 9

what is ELISA?

Answer 9

enzyme-linked immunosorbent assay = lab technique used for detecting and quantifying specific antigens/ antibodies/ proteins in a sample

operates on antigen-antibody interactions, uses enzymes to produce a detectable signal