What is a sputum routine microscopy (SRM) test?

Sputum Routine Microscopy Test

Sputum routine microscopy (SRM) is a laboratory test that involves microscopic examination of a Gram-stained sputum sample to identify bacterial pathogens causing lower respiratory tract infections, primarily used to guide initial antibiotic selection in pneumonia. 1

What the Test Involves

Specimen Collection and Processing

• A deep-cough specimen must be obtained before antibiotic therapy is initiated, then rapidly transported and processed in the laboratory within a few hours of collection 1
• The specimen should be mucopurulent material selected by gross inspection, representing true lower respiratory tract secretions rather than saliva 2
• In office practice, a slide may be prepared, air-dried, and heat-fixed for subsequent interpretation even if immediate Gram staining is not feasible 1

Quality Screening (Cytological Criteria)

The specimen must meet strict cytological criteria to be considered valid for interpretation:

• ≥25 polymorphonuclear cells (PMN) AND <10 squamous epithelial cells (SEC) per low-power field (100x magnification) 1, 3
• Some authorities use a criterion of >10 WBC per SEC 1
• Invalid specimens (≥10 squamous epithelial cells and ≤25 polymorphonuclear cells per field) should be rejected and not examined further, as they represent upper airway contamination rather than lower respiratory secretions 1, 3
• Only 39-41% of pneumonia patients can produce adequate quality specimens by these criteria 1, 3

Microscopic Examination

• Gram staining is performed on specimens meeting cytological criteria, examining for the relative number and types of bacteria present 1
• The test looks for a predominant bacterial morphotype (≥90% of organisms), which significantly increases diagnostic accuracy 1, 3
• Lancet-shaped gram-positive diplococci suggest Streptococcus pneumoniae, the most common cause of community-acquired pneumonia 1
• The first 100 microscopic fields are typically examined, as 99.6% of positive findings are detected within this range 4

Diagnostic Performance

Sensitivity and Specificity

• For pneumococcal pneumonia, sensitivity is 50-60% and specificity is >80% when a predominant morphotype is identified 1
• When good-quality sputum shows a single or preponderant morphotype, sensitivity for S. pneumoniae is 35.4% with specificity of 96.7% 1, 3
• For Haemophilus influenzae, sensitivity is 42.8% with specificity of 99.4% 1, 3
• In one prospective study, Gram stain predicted the blood culture isolate in 85% of valid specimens, allowing appropriate antibiotic selection in >90% of patients 1

Clinical Utility

The test is recommended for hospitalized patients with community-acquired pneumonia to guide initial antimicrobial therapy selection, provided specimens are obtained before antibiotics and properly processed 1

• The test is particularly reliable for Haemophilus pneumonia due to profuse organism numbers 1
• Finding many WBCs with no bacteria reliably excludes infection by most ordinary bacterial pathogens in patients not yet on antibiotics 1
• Results are usually available within hours, unlike culture which takes 24-48 hours 1

Important Limitations and Pitfalls

Technical Limitations

• The validity of Gram stain interpretation is directly related to the experience of the observer, with potential interobserver variability 1
• Laboratory quality control is essential due to variability in interpretation between technicians 1, 3
• Many pneumonia patients cannot produce sputum, particularly older patients 1, 3

Pathogen-Specific Exceptions

• Mycobacteria and Legionella species are exceptions where microscopic criteria may yield misleading results and should not be evaluated using standard cytological screening 1
• Atypical pathogens (Mycoplasma, Chlamydophila) cannot be visualized on routine Gram stain 3

Pre-analytical Factors

• Prior antibiotic administration significantly reduces diagnostic yield of both Gram stain and culture 1, 3
• Delays in processing the specimen compromise results 1
• Even with bacteremic pneumococcal pneumonia, sputum cultures are positive in only 40-50% of cases using standard techniques 1, 3

Complementary Testing

• Routine sputum microscopy should be accompanied by aerobic culture of specimens meeting cytological criteria, with semi-quantitative results reported 1
• Blood cultures should also be obtained from hospitalized patients 1
• For severe disease, urinary antigen tests for S. pneumoniae and Legionella pneumophila should be added where available 1