A 65 year old man has been admitted to your Intensive Care Unit with a presumptive diagnosis of community acquired pneumonia. He is sedated, intubated and ventilated, and is haemodynamically stable.

(b) What specific investigations would you order?  Discuss why.

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College Answer

(b)       What specific investigations would you order?  Discuss why.

Standard CXR to help delineate areas involved (possibly help with aetiology), and serve as baseline. Blood cultures (diagnose organism), and full blood examination (ideally with film to assess white cell morphology; white cell count may be high, low or normal).  Electrolytes including Creatinine (renal impairment, modify drugs) and liver function (organ involvement, modify drugs, help with aetiology). Gram stain (controversial) may help guide therapy, as may sputum culture. Pleural fluid should be tapped (for organism).  Legionella urinary antigen (and/or sputum immunofluorescence) may help confirm diagnosis.  Bronchoscopy and lavage or protected brush specimen may also help in aetiology.

 

Discussion

(b)       What specific investigations would you order?  Discuss why.

Of the following list of tests, the normal font identifies the college canon, and the  italicised ones were added by the deranged author.

  • CXR
  • Arterial blood gas
  • Blood cultures
  • FBC (for leucocyte morphology rather than their number)
  • EUC (to anticipate renal clearance problems)
  • LFTs (to anticipate hepatic clearance problems)
  • Blood culture
  • Sputum culture
  • Pleural fluid culture
  • Legionella urinary antigen
  • Pneumococcal urinary antigen
  • Bronchoscopic lavage specimen for culture
  • Respiratory viral swabs (NAT)
  • Inflammatory markers, eg. CRP and procalcitonin

The evidence and utility of these investigations can be examined in some greater detail:

  • CXR : according to the IDSA and BTS, "a demonstrable infiltrate by chest radiograph or other imaging technique... is required for the diagnosis of pneumonia".
  • ABG: recommended by the BTS for all patients receiving "emergency oxygen". In studies of random mixes of patients, ABG increased the chances of detecting clinically significant hypoxia. It is frightening to think that doctors need an ABG to accurately identify patients with clinically significant hypoxia, but in reality the blood gas result may sway one's decision regarding oxygen supplementation and intubation (eg. a patient who is tachypnoeic, and becoming hypercapnoeic due to fatigue).
  • Blood cultures: all the guidelines recommend that patients with pneumonia of ICU-level severity should get cultured. The specific caveat is that cultures should be collected "whenever the result is likely to change individual antibiotic management". The de-escalation of broad spectrum antibiotics would represent such a change.
  • Sputum culture is neither sensitive nor specific, but the BTS and IDSA recommend that we send purulent samples. Of particular interest are tracheal aspirate and BAL samples (as they might actually be representative of the infectious agent)
  • Pleural fluid culture is valuable if the patient already has empyema, or if one is suspected. This becomes of more interest in the pneumonia which fails to resolve.
  • Pneumococcal urinary antigen (specifically pneumococcal capsular polysaccharide antigen) has broad popularity and multiple advantages:
    • it has good sensitivity and specificity
    • it is not affected by prior antibiotic therapy (result is positive for up to a week after the commencement of antibiotic therapy)
    • its level is higher in more unwell patients
    • it is available earlier than cultures and Gram stains
  • Legionella urinary antigen only detects Legionella pneumophila, whereas there are about 19 other species which act as pathogens in man (particularly immunocompromised man). However, it is also positive more often in patients with severe infection. The assay occasionally cross-reacts with Campylobacter antigens.
  • FBC: the college recommends we look to the toxic granulation of neutrophils in order to make a diagnosis of infection, remarking that the WCC itself could be low, high, or normal. The BTS expand on this further in their Section 5.8 (iii19), quoting a 1990 paper by Holberg et al.  Apparently, among 418 adult Scandinavians, a WCC in excess of 15 was strongly associated with a bacterial (and specifically pneumococcal) pneumonia. It is unclear how this can be generalised to the typical 2015 Western Sydney scenario (150kg bong-smoking diabetic with sleep apnoea) ...but the FBC is probably still important.
  • EUC: not only to "modify drugs", as the college suggests. Renal function plays a role in the stratification of mortality risk (the CURB65 score), which in turn recommends for or against ICU admission.
  • LFTs are mentioned by the college because "organ involvement, modify drugs, help with aetiology". This is fairly accurate.  The BTS guideline mentions LFTs as a means of identifying Legionella cases (which tend to have deranged LFTs). Similarly, leptospirosis presents with a degree of hepatitis.   
  • Respiratory viral swabs (NAT) - locally, the viral swabs are tested (PCR) for several clinically important viruses, including influenza A and B, parainfluenza, RSV, rhinovirus, adenovirus and possibly one or two others. The BTS suggest that this PCR approach is superior to waiting for serological tests
  •  Inflammatory markers, eg. CRP and procalcitonin  are discussed in the BTS and ISDA guidelines. Salient points made by the societies include the following:
    • CRP which fails to fall by 50% byy day 3-4 is associated with an increased 30-day mortality
    •  CRP levels over 100 were 96% sensitive in identifying pneumonia in one poorly-designed study
    • Procalcitonin seems to be a sensitive marker of severe disease and bacterial (especially pneumococcal) aetiology.

 

References

References

Metlay, Joshua P., Wishwa N. Kapoor, and Michael J. Fine. quot;Does this patient have community-acquired pneumonia?: Diagnosing pneumonia by history and physical examination." Jama 278.17 (1997): 1440-1445.

Mandell, Lionel A., et al. "Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults." Clinical infectious diseases 44.Supplement 2 (2007): S27-S72.

Blum, Claudine Angela, et al. "Adjunct prednisone therapy for patients with community-acquired pneumonia: a multicentre, double-blind, randomised, placebo-controlled trial." The Lancet 385.9977 (2015): 1511-1518.

Lim, W. S., et al. "British Thoracic Society community acquired pneumonia guideline and the NICE pneumonia guideline: how they fit together." Thorax (2015): thoraxjnl-2015.

Lim, Wei Shen, et al. "BTS guidelines for the management of community acquired pneumonia in adults: update 2009." Thorax 64.Suppl 3 (2009): iii1-iii55.

Eccles, Sinan, et al. "Diagnosis and management of community and hospital acquired pneumonia in adults: summary of NICE guidance." BMJ 349 (2014): g6722.

The actual NICE recommendations (2014)

Lim, W. S., et al. "Defining community acquired pneumonia severity on presentation to hospital: an international derivation and validation study." Thorax 58.5 (2003): 377-382.

Levin, Kenneth P., et al. "Arterial Blood Gas and Pulse Oximetry in Initial Management of Patients with Community‐acquired Pneumonia." Journal of general internal medicine 16.9 (2001): 590-598.

Venkatesan, P., and J. T. Macfarlane. "Role of pneumococcal antigen in the diagnosis of pneumococcal pneumonia." Thorax 47.5 (1992): 329-331.

Muder, Robert R., and L. Yu Victor. "Infection due to Legionella species other than L. pneumophila." Clinical infectious diseases 35.8 (2002): 990-998.

Johansson, Niclas, et al. "Procalcitonin levels in community-acquired pneumonia-correlation with aetiology and severity." Scandinavian journal of infectious diseases 46.11 (2014): 787-791.