Bacterial sputum culture + the rouleaux formation

Thank you to Chris Berger for presenting a fascinating case of a 26 y/o man with a structural lung defect who presented with a recurrent pneumonia symptoms who was found to have a pulmonary mycetoma with overlying community acquire pneumonia
 When is a bacterial sputum culture useful in patients with pneumonia?
  • There are three issues that limit the utility of bacterial sputum cultures
    • sensitivity varies widely so false negatives are common, especially with fastidious organisms like mycoplasma pneumonia and h flu
    • specificity varies widely because easily grown organisms like MRSA and GNRs can be airway contaminants
    • most mild, outpatient CAP resolves with empiric antibiotic therapy. So from a resource-utilization standpoint, it’s not necessary to send sputum culture in every patient.
  • The Infectious Disease Society of America (IDSA) recommends sputum cultures (and other microbe-specific diagnostic testing) in the following circumstances
IDSA table sputum culture indications.png
  • Bacterial sputum cultures are always recommended in HCAP and VAP as well as non-resolving or recurrent pneumonia.
  • The IDSA also points out that true pathogens should be present in moderate to heavy amounts on the gram stain and culture.
    • hard to grow organisms that rarely colonize the airways are considered pathogenic in any quantity. Some examples include mycobacterium tuberculosis, mycoplasma pneumoniae, Chlamydia psittaci and pneumonae
A bonus pearl on the rouleaux formation vs agglutination
  • in agglutination, RBCs stick together in 3 dimensional shapes. It is caused by
    • immune-mediated thrombocytopenia
    • EDTA-mediated pseudoagglutination (an artifact)
    • certain medications
  • In rouleaux, RBCs form stacks, like a stack of coins.
    • rouleaux is caused by high levels of immunoglobulins (as in multiple myeloma and occasionally polyclonal gammopathy) and fibrinogen.
    • this is a normal finding in healthy horses. I know you were worried about all that horse anemia.
  • They can look similar on a smear! to distinguish the two, perform a saline dilution. Rouleaux will resolve with saline dilution, but agglutination will not, unless the antibodies are low-avidity.
Bain BJ Diagnosis from the blood smear. N Engl J Med. 2005;353(5):498.
Mandell LA1Wunderink RGAnzueto ABartlett JGCampbell GDDean NCDowell SFFile TM JrMusher DMNiederman MSTorres AWhitney CGInfectious Diseases Society of AmericaAmerican Thoracic Society. Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clin Infect Dis. 2007 Mar 1;44 Suppl 2:S27-72.
Not usually my style, but a super helpful website on reading blood smears compiled by a veterinarian pathologist:

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