Microbiological Diagnosis of LRTIs Flashcards
What are the main diagnostic techniques?
Microscopy and culture of sputum and blood
Antigen detection methods
PCR
Serology (Ab measurement)
What is done with sputum and blood cultures?
Sputum:
Gram stain sputum for organisms and pus cells
Culture for major respiratory pathogens (overnight) (Strep pneumoniae, H flu, moraxella catarrhalis)
Blood cultures:
Taken from any patient with severe sepsis
Many patients with pneumonia have bacteraemia
Too few organisms to see in microscopy
Gram stain done for first identification, to commence treatment, further identification then done
What are examples of organisms not easily cultures?
Legionella pneumophilia
Mycoplasma pneumoniae
Chlamydia psittaci
Coxiella burnetti
What is a further technique performed to sample lower airways?
Broncho-alveolar lavage
Catheter aspirate in ventilated patient
- Less liable to contamination
- Used for diagnosis of ventilator associated pneumonia
What tests are done in suspected TB?
ZN or auramine phenol stain
Acid and alcohol fast bacilli (AAFB)
Extended culture (weeks for mycobacteria)
Full ID as M. TB only possible once it has grown
What is antigen detection?
Non-cultural methods of demonstrating the presence of an organism
- detect specific antigen immunologically
Legionella and pneumococcal antigen can be detected in urine, viruses in nasopharyngeal secretions.
Latex agglutination can also done - latex particles coated with monoclonal Abs specific for organism in question
ELISA as well
How is antibody detection in serological diagnosis done?
Measure host antibody response to organism
Usually difficult to culture or detect the organism directly
Previously used for viral infections and so called atypical causes of pneumonia.
Now mostly replaced by PCR
IgM is a marker of current infection
IgG represents previous infection
Total Ab increase during course of illness?
