182a - TB and Viral Infection Testing Flashcards
Which of the following is not used for clinical diagnosis?
- Virus culture
- Serology
- Rapid antigen detection
- Molecular methods
b. Serology
Molecular methods (PCR) are preferred
What are the strengths of molecular assays (PCR) over other diagnostic methods?
-
Better performance
- Sensitivity and specificity
- Faster result time
- Detection of non-culturable viruses
- Detection of more coinfections
- Automation
- Potential for quantification
What are the limitations of the PPD skin test for tuberculosis?
- False positives
- BCG vaccinees
- Repeat PPD testing
- Other mycobacterial infection
- Return visit to physician’s office required within 48-72h
- Innacuracies in the placement and interpretation of the test
What are the US recommendations for the BCG vaccine?
Administer to…
- TST-negative individuals residing in areas with annual TB incidence of > 1%
- TST-negative children exposed to individuals with active TB and MDR-TB and unable to take oral isoniazid
The vaccine is protective against systemic and meningeal TB, but not pulmonary TB
____ and ______ signaling are crucial for controlling mycobacterial infections (keeping them in a latent form)
IFN-gamma** and **IL-12 signaling are crucial for controlling mycobacterial infections (keeping them in a latent form)
How does the IGRA overcome the problem of false-positives due to BCG vaccination?
The IGRA uses antigens that are not present in the BCG sub-strains of M. bovis.
(ESAT-6, CFP-10, TB-7.7)
Why is multiplexing so useful for the diagnosis of infectious syndromes?
Similar respiratory syndromes can be caused by many different viruses or bacteria
Multiplexing allows us to detect the presence of several different viruses with one test
If a patient has a LOF mutation in their STAT1 IFN-gamma receptor, which infections are they more susceptible to?
Why?
Mycobacterial infections
The IFN-gamma signalling pathway is important for the control of these infections.
A LOF mutation in the STAT1 IFN-gamma receptor prevents this pathway from working
Which respiratory virus is most likely to present year-round?
- RSV
- Influenza
- Adenovirus
c. Adenovirus
RSV and influenza present more seasonally
Which pathogens have reduced sensitivity in multiplex testing?
Adenovirus
Bordetella pertussis
Is viral culture used clinically for diagnosis?
Why or why not?
Used to be the gold standard, but is not longer recommended as a primary method for respiratory virus diagnostic testing
It takes too long to inform clinical management
Lateral flow immunoassay and DFA are examples of what kind of diagnostic method?
Rapid antigen detection
What is the threshold for a positive PPD-skin test (TST) in the United States?
Depends on pre-test probability
The following induration diameters qualify as positive:
- ≥ 5 mm
- If the patient has HIV and is immunosuppressed
- If the patient has had close contact with a known TB patient
- ≥ 10 mm
- If TB exposure is suspected
- If the patient is a resident of a TB-endemic area
- ≥ 15 mm
- If the patient has no known risk factors
What are the recommendations for latent TB testing?
- IGRA is preferred but TST is acceptable
- Except for immigration screening: TST is no longer used
What are the limitations of molecular assays (PCR)?
- Higher cost
- Require specific target ordering
- For single target assays
- Cannot provide info about organism viability
- => Cannot test to see if the patient is cured
What are the limitations of multiplex detection?
- Assay performance
- Reduced sensitivity for adenovirus and B**ordetella pertussis
- High cost
Increased phosphorlyation of the STAT1 IFN-gamma receptor may indicate the presence of which infection?
Mycobacterium tuberculosis
IFN-gamma signalling works to control the bacteria inside of granulomas
Phosphorylation of the STAT1 IFN-gamma receptor is part of this pathway
What is the difference between ELISA and ELISPOT testing for TB?
ELISA = QunterFERON assay (IGRA)
ELISPOT = T-Spot (the technically challenging, difficult to interpret one)
Deficiency of which signaling pathway enhances a patient’s susceptibility to mycobacterial infections?
The IL-12 / IFN-gamma signaling axis
This axis regulates the CD4 Th1 T cell respose that helps us control mycobacterial infections
What is the threshold for a positive result on the ELISPOT assay?
- Positive result: ≥ 8 spots
- Negative result: ≤ 4 spots
- Inconclusive: 5-7 spots
Describe the signaling pathway that controls mycobacterial infections (ex: keeps them in a latent form)
- Dendritic cells and macrophages produce IL-12
- IL-12 activates naive CD4 T cells
- Activated CD4 T cells differentiate into Th1 CD4 T cells
- Th1 CD4 T cells release IFN-gamma
- IFN-gamma causes…
- Signaling to continue IL-12 secretion
- Increased transcription of IFN-stimulated genes via the STAT1 IFN-gamma receptor
- Expression of these genes causes granulomas to form around the TB infection to control it
What are the advantages and disadvantages of rapid antigen detection for viruses?
- Advantages
- Fast
- Easy to use
- Low cost
- Disadvantage
- Limited sensitivity (10-90%)
Less commonly used, PCR is preferred
How is latent vs active TB distinguished?
TST and IGRA cannot tell
Use microbiological, clinical, radiological, histopathological evidence
In a TST, which diameter of the induration should you measure?
Transverse
(False positive more likely with longitudinal)
What are the important glycoproteins of influenza?
- Hemaglutinin (HA)
- Involved in the attachemtn fo virus to cells and initiation of infection
- Neuraminidase (NA)
- Facilitates the release of new virions from infected cells
What are the advantages of the QuantiFERON-TB assay over the TST?
- Unaffected by previous BCG vaccination
- Does not with environemental mycobacteria
- But will react with some other mycobacteria
- Requires only 1 visit
- Multiple tests do not cause false positive results
- Results available in 24h
When should specimens be tested to test for viruses?
ASAP
- <5 days after symptom onset
- Within 48h is preferred
- No later than 7 days after symptom onset
Who are the best candidates for multiplex testing?
- Serious infections
- Immunocompromised patients
- Patients with cancer
- Inpatients
Shouldn’t be used for everyone - high cost
The BCG vaccine protects against _____________ TB, but not _______________ TB
The BCG vaccine protects against systemic and meningeal** TB, but not **pulmonary TB
Which method of diagnostic testing for viruses can be automated?
Molecular testing (PCR)
(GeneXpert, BD MAX)
What is the current reference method for diagnosing viral infections?
Molecular assay (PCR)
What kind of cells produce IL-12?
Dendritic cells and macrophages
What are the 4 tubes used in the QuantiFERON Gold assay?
- Positive control
- Negative control
- CD4 T cell response
- CD8 T cell response
What are the advantages of molecular assays (PCR) for diagnosing viral infections?
Fast
High sensitivity
High specificity
