- Over reliance on negative culture results can dismiss a diagnosis of lower respiratory tract infection (LRTI) when the microscopy on the same sample indicates bacteria are present!
- Over reliance on positive culture results can lead to unnecessary antibiotic prescriptions when the culture result actually identifies a contaminant
The approach to interpreting a sputum result correctly is to look at the report as a whole, after all if the laboratory didn’t think all of the information on the report was necessary they
wouldn’t be telling you it!
The report is divided into three sections: Appearance, Microscopy and Culture. These should be looked at in this order.
stained and determines if the micro-organism that grows is the likely cause of an infection of the LRT.
infection and inappropriate antibiotic usage.
• Mucoid samples DO NOT contain white blood cells and therefore DO NOT indicate infection. If a micro-organism grows it will be a contaminant from the URT flora. The danger with mucoid samples is with immunodeficient patients who are unable to produce white blood cells to form a purulent sample e.g. neutropaenic patients.
• Purulent samples DO contain white blood cells which indicate inflammation and may be consistent with infection. However, it is possible to have inflammation without infection e.g. in asthma. If a micro-organism grows it is likely to be the cause of the LRT infection.
• Blood stained samples indicate lung destruction and this can be consistent with infection, but it is not the only possible diagnosis as other processes can cause similar lung damage e.g. pulmonary embolism, cancer and vasculitis. If
a micro-organism grows it is likely to be the cause of
the LRT infection.
growing. This is helpful as it can indicate infection in a negative culture e.g. when current antibiotics have stopped the micro-organism growing or when the micro-organism is very slow growing.
Some laboratories still do Gram stains on respiratory samples and these can be helpful if a pure and predominant
micro-organism is seen, the report states either Gram-negative or Gram-positive coccus or bacillus. The process is often now reserved for samples taken directly from within the chest such as a bronchoalveolar lavage (BAL).
If tuberculosis is suspected then a Ziehl-Neelsen stain is performed to look for the presence of acid fast bacilli (Mycobacteria sp.), this report will state ZN positive or negative.
The culture result should only be looked at AFTER a decision has been made about the likelihood of LRTI as the diagnosis is based upon (a) patient history, examination and chest x-ray (b) the reported Appearance and Microscopy of the sputum sample and (c) the risk of contamination of the sputum sample - see Appearance.
THINK! Does the micro-organism that has been grown actually cause respiratory infections e.g. Streptococcus pneumoniae is the most common bacteria isolated in
pneumonia, E. coli is not a cause of community acquired pneumonia but may be more important in ventilator associated pneumonia. However, Neisseria sp. live happily as part of the URT flora but do not cause LRTI and so do not need treating. It is important to know the common micro-organisms which cause LRTI in order to judge the significance of the culture result.
BE CAREFUL! Prior antibiotic use can alter the flora of the URT, e.g. a patient treated with Amoxicillin is UNLIKELY to grow Streptococcus pneumoniae or Haemophilus influenzae, the two most common bacterial causes of pneumonia, because they are both sensitive to Amoxicillin. However, Klebsiella pneumoniae, which is inherently resistant to Amoxicillin, will happily replace them in the patient’s URT and if grown is very likely to be a contaminant and a consequence of altered flora. Klebsiella pneumoniae is a very rare cause of pneumonia (Friedlander’s pneumonia, a type of necrotising pneumonia, the patient will be very ill and cough out “redcurrant jelly” sputum).
To diagnose pneumonia you need take a careful history, examine the patient and if in hospital look at a chest x-ray. Once the diagnosis is made the lab report on the sputum sample can help decide what micro-organism might be causing the pneumonia. Remember, in order to interpret the result correctly the whole result (not just the culture) requires consideration:
1) Appearance to check for inflammation, WBC and risk of contamination,
2) Microscopy to look for the presence of micro-organisms, 3) THEN Culture to identify the micro-organism and what antibiotics it’s sensitive to.
Getting the right information from a report allows you to treat patients more effectively.