Microbiology Cheat Sheet
Specimen to Test to Interpretation
Specimen to test to interpretation for NEET-PG: blood culture sets, CSF handling, sputum quality, transport media, and choosing microscopy, culture, or PCR.
MedNext Academy | 3 min read
Specimen to Test to Interpretation
Specimen to test to interpretation for NEET-PG: blood culture sets, CSF handling, sputum quality, transport media, and choosing microscopy, culture, or PCR.
This topic covers the principles of specimen collection and transport, appropriate laboratory test selection, and correct interpretation of microbiological results.
High-yield lines
- The quality of the specimen determines the quality of the diagnosis, so collection from the true site of infection is critical.
- Specimens should be collected before antibiotics whenever possible, in adequate quantity, and transported promptly or in appropriate transport media.
- Blood culture sensitivity rises with the number of sets: about 73 percent for 1 set, about 90 percent for 2 sets, and over 99 percent for 3 sets.
- For adults, 20 to 30 millilitres of blood per culture set from separate venepuncture sites maximises yield and helps distinguish contamination.
- CSF must never be refrigerated because cooling kills fastidious organisms such as Neisseria meningitidis.
- Urine should be midstream clean-catch or from a catheter port, and must reach the laboratory within 2 hours or be refrigerated or preserved with boric acid.
- Deep aspirate or tissue is preferred over surface swabs, with anaerobic transport media used for deep specimens.
- Direct detection by microscopy, antigen tests, or PCR is ideal for rapid acute decisions, while culture is the gold standard allowing susceptibility testing.
- Serology is used for difficult-to-culture organisms, ideally with paired acute and convalescent sera to demonstrate seroconversion.
- Molecular methods (PCR and NAAT) offer high sensitivity and specificity, detect resistance genes, but also detect dead organisms.
- Sputum quality is assessed by squamous epithelial cell and neutrophil counts, and salivary samples should be rejected and recollected.
- Stool for parasitology should be placed in 10 percent formalin to preserve cysts and ova, and in PVA fixative for permanent stains.
- Every clinical specimen should be labelled at the bedside immediately, and mislabelled specimens should be rejected and recollected.
- Only tests that will change management should be ordered, and complete clinical information must accompany the request form.
Mapped competency codes
- MI8.9
- MI8.10
- MI8.13
- MI8.15
Continue into the full chapter
This summary maps to MI8-zoonotic-diseases-and-miscellaneous.
Frequently Asked Questions
How many blood culture sets are needed for reliable sensitivity?
At least two sets from separate sites give about 90 percent sensitivity, and three sets exceed 99 percent.
Why must CSF never be refrigerated?
Refrigeration kills fastidious organisms such as Neisseria meningitidis and Streptococcus pneumoniae, reducing culture yield.
What determines the reliability of a sputum culture?
Specimen adequacy: acceptable sputum has fewer than 10 squamous epithelial cells and more than 25 neutrophils per low-power field.
When should serology use paired sera?
For difficult-to-culture organisms, paired acute and convalescent sera demonstrate a diagnostic fourfold rise in antibody titre (seroconversion).
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