Community Acquired Pneumonia
Clinicians
Introduction
This section is for actively practising physicians. However, anyone is free to read this although caution is advised especially if you are a beginner! I have structured this section under the headings outlined in the black and white (B&W) section for ease of understanding. For the sake of efficiency, I will not be repeating the basics and if you are in doubt please go back to the B&W section for a quick catch-up.
1. Aetiology
Role of biological samples in CAP
Use the context and demographics to make an informed guess of the likely aetiology as before. Recall that microbiological testing is more useful in the following situations:
- Patients are not improving despite seemingly appropriate antibiotics.
- Specific high risk epidemiological conditions like influenza, MERS, SARS etc.
- In the case of severe community acquired pneumonia (CAP), to switch to narrow spectrum antibiotics.
- All patients with hospital acquired pneumonia.
- Immunosuppressed individuals.
Figure 1 shows the role of biological samples in CAP.
Figure 1
Some interesting observations:
- Incidence of pneumococcal pneumonia is declining because of pneumococcal vaccinations.
- Nearly a third of CAPs are caused by viruses (Jain et al., 2015).
- No microbial pathogens identified in more than half of patients despite newer extensive molecular testing. The lung microbiome (blog) might bridge this gap in our knowledge in telling us more about the pathogenesis of lung infections.
- In Asian countries, Klebsiella is one of the top three causes of CAP.
Procalcitonin
It is impossible to discriminate between bacterial and viral infections in a given patient. This is because of two reasons: first, culturing the bacteria takes time and second, we often don’t identify any microbes at all! Given these problems a newer biomarker called procalcitonin is proving helpful. In health, it is only produced by thyroid neuroendocrine cells which is then cleaved to calcitonin and released. Therefore normally, procalcitonin is virtually undetectable.
Figure 2 shows the value of procalcitonin in CAP.
Figure 2
It is important to note in the figure, that there are many false negatives. The test can also be falsely elevated in any systemic stress like burns, chronic renal insufficiency and neuroendocrine lung and thyroid medullary malignancies. Mycobacterial infections can unhelpfully cause a high or low level, making it useless in this setting. Bottomline: use clinical judgement as always!
2. Clinical diagnosis
Making the diagnosis
Ready for a quick reality check?
You rarely see the classic descriptions of sputum colour outside exam settings. Elderly and immunosuppressed rarely mount an immune response and consequently rarely have fever or other vital sign abnormality. “So how can we diagnose pneumonia in them?” I hear you cry. Well, we turn to our gold standard, the chest x-ray (CXR)! Compatible ‘acute’ history, sketchy signs and compatible CXR help us. However, we need to remain open to other possibilities if things don’t go our way. That’s why real life is more exciting and fun.
The simple rule of thumb is, the greater the number of abnormal signs of consolidation that you find, the greater is the likelihood of pneumonia. This is true of any diagnosis in medicine!
The same can be said of tests. A positive CXR raises the probability of pneumonia. A negative test decreases the probability of pneumonia.
Therefore, if the CXR is negative in someone with a high suspicion of pneumonia you will still need to treat them as pneumonia and repeat the CXR in 24-48 hours.
Pearl of wisdom: The absence of any abnormal vital sign, especially respiratory rate < 20/min and lack of fever, decreases the probability of pneumonia to < 1% (Gennis et al., 1989). You may as well skip getting a CXR in this instance if pneumonia is what you were trying to exclude!
Non-resolving pneumonia
What happens when we have diagnosed pneumonia and despite treatment, things don’t improve? Unsurprisingly, we call this non-resolving pneumonia. Figure 3 shows the differentials you will have to entertain with some examples underneath each possibility. Think wide! A good general physician only requires breadth of knowledge not depth.
Figure 3
3. Antibiotic therapy
In the setting of sepsis, ensure that antibiotics are initiated within one hour (NICE link). Remember that every hour of delay increases mortality by 8%.
There are two ways of deciding on which antibiotic to use in a given patient with CAP: pneumonia severity scores (like CURB-65 used in UK or pneumonia severity index) or site of care i.e. out-patients, in-patients or ICU (used in US). Both these approaches have pros and cons which are highlighted in table 1 below.
Table 1: Antibiotic choice based on severity score or site of care
| Severity scores | Site of care | |
|---|---|---|
|
Pros |
Simple to use especially for non-respiratory physicians and less experienced doctors. |
More rational and takes local prevalence into account |
|
Cons |
Under or over-estimation of the severity of CAP.
|
Wide variation in regional practice as to where the patients are treated i.e. in or out-patient or ICU.
Over-use of broad spectrum antibiotics.
|
How do we critically evaluate the guidelines?
Let us consider the setting for antibiotic decisions based on severity scores.
- The most common bacterial causes of CAP are Streptococcus pneumoniae, atypical pathogens, Haemophilus influenzae and gram negative enterobacteriaceae (famous member Klebsiella) in that order.
- Atypical pathogens are more common in the low severity pneumonias particularly in the out-patient setting.
With this in the background, the use of oral amoxicillin monotherapy in low severity CAP as per BTS guidelines would inevitably fail to cover atypical pathogens that occur in this group of patients.
On the other hand, Legionella pneumonia is equally found in both out-patient and in-patient populations. Therefore treatment regimens based on site of care will again provide incomplete cover against atypical pneumonia.
Background and demographics predict microbial aetiology more than severity scores (Singanayagam and Chalmers, 2013). The European Respiratory Society guidelines recommend that the antibiotic choice should be based on 10 different criteria. The severity score is just one of these criteria in addition to other factors like co-morbidities, immunosuppression, aspiration risk factors etc (Woodhead et al., 2011).
Ultimately it is clinical judgement including recognition of an ill patient that helps in selecting the right antibiotic. We can use severity scores as a second opinion to our clinical judgement but not replace it.
National and regional microbial resistance patterns also affect antibiotic choice. Do you want to have a look at the world-wide microbial resistance patterns? Look no further, it is right here (Global antibiotic resitance map) at your finger-tips.
When do we discharge patients?
Ensure that social factors have been looked into and that there have been no vital sign abnormalities. You do not need to observe patients overnight when IV antibiotics are switched to oral, which is traditionally done (Nathan et al., 2006).
What is the duration of treatment?
There are no clear studies to help us here. The common practice is a minimum of 5 days extended further depending on the severity of pneumonia, co-morbidities and any complications. Ensure that they are afebrile in the last 48-72 hours with no more than one vital sign abnormality. Some centres use procalcitonin and CRP to help guide this decision. Remember that changes in these tests lag 48 hours behind the clinical picture. Exercise clinical judgement as always.
Radiographic resolution
- Patients aged > 50 years, male and smokers in particular should have a follow up CXR in 8-12 weeks.
- Radiographic resolution in elderly, with underlying lung disease and multi-lobar involvement can take much longer.
- Incidence of lung cancer after pneumonia is very low – 1% in 3 months and 2% over 5 years but varies with the presence of risk factors as above (Tang et al., 2011).
References
Gennis, P. et al. (1989) ‘Clinical criteria for the detection of pneumonia in adults: guidelines for ordering chest roentgenograms in the emergency department.’, The Journal of emergency medicine, 7(3), pp. 263–8.
Jain, S. et al. (2015) ‘Community-Acquired Pneumonia Requiring Hospitalization among U.S. Adults.’, The New England journal of medicine, 373(5), pp. 415–27. doi: 10.1056/NEJMoa1500245.
Nathan, R. V et al. (2006) ‘In-hospital observation after antibiotic switch in pneumonia: a national evaluation.’, The American journal of medicine, 119(6), pp. 512.e1–7. doi: 10.1016/j.amjmed.2005.09.012.
Singanayagam, A. and Chalmers, J. D. (2013) ‘Severity assessment scores to guide empirical use of antibiotics in community acquired pneumonia’, The Lancet Respiratory Medicine, 1(8), pp. 653–662. doi: 10.1016/S2213-2600(13)70084-5.
Tang, K. L. et al. (2011) ‘Incidence, correlates, and chest radiographic yield of new lung cancer diagnosis in 3398 patients with pneumonia.’, Archives of internal medicine, 171(13), pp. 1193–8. doi: 10.1001/archinternmed.2011.155.
Woodhead, M. et al. (2011) ‘Guidelines for the management of adult lower respiratory tract infections – Full version’, Clinical Microbiology and Infection, 17, pp. E1–E59. doi: 10.1111/j.1469-0691.2011.03672.x.
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