Reading a sputum culture result correctly requires understanding the Gram stain quality assessment, the significance of the organism grown in the context of the clinical picture, the distinction between colonisation and infection, and when the result should influence prescribing.
The Sputum Quality Assessment
Before any organism grown in sputum culture can be interpreted meaningfully, the quality of the sample must be assessed. The standard quality criterion is the microscopic cellular composition of the sputum Gram stain: the Murray-Washington (or Bartlett) scoring system.
In the Murray-Washington system, the number of polymorphonuclear leucocytes (PMNLs, neutrophils) and squamous epithelial cells (SECs, from the oropharynx and upper airways) per low-power field (LPF, 100x magnification) are counted.
An acceptable sample (valid for culture) has: more than 25 PMNLs per LPF AND fewer than 10 SECs per LPF. Many PMNLs indicate a sample from an actively inflamed lower respiratory site. Few SECs indicate minimal oral contamination.
A reject-grade sample: more than 25 SECs per LPF (predominantly oropharyngeal in origin). Most laboratories reject or flag these samples and request repeat collection, as culturing them produces misleading results from oral flora.
Intermediate samples (10 to 25 SECs): may be processed with a note that they represent a suboptimal specimen.
The Gram stain also provides the first microbiological information: organism morphology seen on Gram stain from a validated sample provides rapid presumptive identification (gram-positive cocci in pairs = pneumococcus, gram-negative rods = Haemophilus or Enterobacteriaceae, gram-negative diplococci = rare but consider Moraxella) and is often the most useful single result for guiding immediate prescribing before culture results are available.
Common Organisms in Sputum and Their Significance
Streptococcus pneumoniae: the most common bacterial cause of community-acquired pneumonia. A sputum culture positive for S. pneumoniae from a quality sample in a patient with clinical and radiological pneumonia is a significant and clinically important result. However, S. pneumoniae dies rapidly in transit (it is fragile) and is fastidious, so negative sputum culture does not exclude pneumococcal pneumonia. Blood cultures and urinary antigen test are the more reliable confirmatory tests.
Haemophilus influenzae: a commensal of the upper respiratory tract in many adults, particularly smokers and those with COPD. Heavy growth of H. influenzae from a quality sputum in an acute COPD exacerbation or in a non-smoker with pneumonia is clinically significant. However, H. influenzae commonly colonises the respiratory tract of COPD patients without causing acute infection, so clinical correlation is essential.
Moraxella catarrhalis: a common cause of acute exacerbations of COPD. Nearly always beta-lactamase-producing and co-amoxiclav or cephalosporin therapy is appropriate.
Staphylococcus aureus: sputum culture positive for S. aureus (including MRSA) following influenza is an important finding: post-influenza staphylococcal pneumonia is well-recognised and associated with high mortality. S. aureus as a sole isolated dominant organism in a sputum from a patient with clinical pneumonia, particularly in the context of influenza or influenza-like illness, is a significant result. S. aureus in a sputum from a bronchiectasis patient may represent chronic colonisation.
Pseudomonas aeruginosa: in patients with bronchiectasis, cystic fibrosis, or severe COPD, chronic Pseudomonas colonisation is common. A positive sputum culture for P. aeruginosa in a CF or bronchiectasis patient requires interpretation in the context of previous cultures (is this a new isolate or chronic coloniser?), clinical status (acute exacerbation vs stable), and susceptibility pattern. Chronic Pseudomonas colonisation in bronchiectasis does not require eradication therapy in stable disease.
Mixed flora (normal respiratory flora): a common sputum culture result, particularly from suboptimal quality samples. This result simply means multiple organisms of the types normally colonising the oropharynx were grown. It provides no clinically useful information and should not prompt antibiotic therapy.
Sputum in Special Populations
Cystic fibrosis (CF): sputum microbiology in CF follows a predictable age-related pattern. Early childhood: S. aureus and H. influenzae. By adolescence: P. aeruginosa colonisation (initially intermittent, then chronic). Advanced disease: Burkholderia cepacia complex (associated with rapid deterioration and high transplant risk), Stenotrophomonas maltophilia, MRSA, Aspergillus fumigatus (colonisation in approximately 15 per cent of CF patients, occasionally causing allergic bronchopulmonary aspergillosis, ABPA). Annual susceptibility testing of P. aeruginosa is essential in CF patients given the high rate of acquired resistance.
Immunocompromised patients: in HIV-positive patients with low CD4 counts, a sputum (or BAL) Grocott methenamine silver stain positive for Pneumocystis jirovecii cysts is diagnostic of PCP. In haematological malignancy patients with neutropenia, BAL for Aspergillus galactomannan antigen, Aspergillus PCR, and culture (Aspergillus fumigatus, A. flavus) are critical investigations for invasive pulmonary aspergillosis.
Non-tuberculous mycobacteria (NTM): Mycobacterium avium complex (MAC), M. kansasii, and M. abscessus are increasingly important respiratory pathogens in patients with structural lung disease (bronchiectasis, COPD, prior TB lung damage). ATS/ERS/ESCMID criteria for NTM pulmonary disease require: at least two positive sputum cultures for the same NTM species, or one positive BAL culture, in the context of compatible clinical and radiological findings.
Frequently Asked Questions
What is a quality sputum sample?
A quality sputum sample for culture has more than 25 polymorphonuclear leucocytes and fewer than 10 squamous epithelial cells per low-power field on Gram stain. More PMNLs indicate the sample contains material from an actively inflamed lower respiratory site. Few SECs indicate minimal contamination with oropharyngeal flora. Samples that fail this criterion are rejected by most laboratories as they would yield misleading results.
Why does mixed oropharyngeal flora on sputum culture not require treatment?
Mixed oropharyngeal flora means the culture has grown a mixture of organisms that normally colonise the mouth and throat. This result indicates the sample was contaminated with upper airway flora during collection and does not represent lower respiratory tract infection. Treating mixed oral flora with antibiotics would be inappropriate and harmful.
What is the difference between colonisation and infection in sputum?
Colonisation means a microorganism is present in the respiratory tract without causing tissue damage or active disease. Infection means the organism is actively causing inflammation, tissue damage, and symptoms. Many organisms (H. influenzae, P. aeruginosa, S. aureus) can colonise the airways without causing infection, particularly in patients with COPD, bronchiectasis, or cystic fibrosis. Clinical correlation — symptoms, fever, worsening lung function — distinguishes colonisation from infection.
What is the significance of S. aureus in sputum after flu?
Post-influenza staphylococcal pneumonia is a well-recognised and serious complication of influenza infection. Influenza damages the respiratory epithelium and impairs mucociliary clearance, allowing S. aureus to superinfect. S. aureus grown from quality sputum in a patient with pneumonia following influenza or influenza-like illness should be treated with appropriate anti-staphylococcal therapy, with MRSA cover if MRSA risk factors are present.
What organisms are associated with cystic fibrosis?
The characteristic microbial evolution in CF lungs: early childhood — S. aureus and H. influenzae; adolescence — P. aeruginosa (initially susceptible, then multidrug-resistant mucoid phenotype); advanced disease — Burkholderia cepacia complex (particularly Burkholderia cenocepacia, associated with cepacia syndrome and high transplant mortality risk), Stenotrophomonas maltophilia, Achromobacter xylosoxidans, MRSA, Aspergillus.
When does H. influenzae in sputum require treatment?
H. influenzae in sputum requires treatment when: it is grown in heavy or dominant growth from a quality sputum sample, in a patient with clinical features of lower respiratory infection (worsening dyspnoea, increased sputum, purulent sputum, fever in COPD exacerbation or pneumonia). H. influenzae in a clinically stable patient's routine sputum monitoring culture is colonisation and does not warrant antibiotic therapy.
What is the significance of Pseudomonas aeruginosa in COPD?
P. aeruginosa in a COPD patient may represent colonisation or acute infection. In severe COPD (FEV1 below 30 per cent predicted) with frequent exacerbations, P. aeruginosa colonisation is common and is associated with worse outcomes. New isolation of P. aeruginosa in an acute COPD exacerbation, or growth in quality sputum from a patient with clinical pneumonia, warrants anti-pseudomonal therapy (piperacillin-tazobactam, ceftazidime, or fluoroquinolone with antipseudomonal activity). Susceptibility testing is essential.
How is NTM pulmonary disease diagnosed?
ATS/IDSA/ERS/ESCMID criteria for NTM pulmonary disease diagnosis require all of: clinical (respiratory symptoms), radiological (nodular or cavitary opacities or multifocal bronchiectasis on CT), AND microbiological (two or more positive sputum cultures for the same NTM species, or one or more positive BAL/bronchoscopic cultures, or positive pleural/lung biopsy culture or histopathology with NTM on culture). Meeting all three criteria is required before initiating treatment.
What is PCP and how is it diagnosed in sputum?
Pneumocystis jirovecii pneumonia (PCP) is an opportunistic fungal infection of immunocompromised patients (HIV with CD4 below 200, solid organ transplant, haematological malignancy). Diagnosis from respiratory samples: induced sputum (higher yield than spontaneous sputum) or BAL stained with Grocott methenamine silver, calcofluor white, or Giemsa shows characteristic cyst forms (8-nucleate asci) or trophozoites. PCR from BAL is more sensitive than microscopy and is increasingly used.
What is bronchiectasis and how does it affect sputum microbiology?
Bronchiectasis is irreversible dilation and distortion of bronchi from recurrent or chronic inflammation and infection. It leads to impaired mucociliary clearance and chronic bacterial colonisation. The most common colonising organisms are H. influenzae, P. aeruginosa, M. catarrhalis, and S. aureus. Patients often have chronic sputum production and frequent exacerbations. Sputum microbiology surveillance guides exacerbation management and identifies emergence of new resistant organisms.