3.1 Prioritization, Rejection, and Specimen Preparation

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Specimen prioritization

CSF is the prototype STAT microbiology specimen. Neisseria meningitidis and Streptococcus pneumoniae die and lose characteristic morphology as the tube sits; neutrophils lyse; and empiric meningitis therapy is rewritten by a smear that should have been stained within minutes of receipt, not after a rack of urines. Other normally sterile fluids—pleural, peritoneal, synovial, pericardial, and vitreous—follow immediately because they are invasive, low-burden, and irreplaceable. Operating-room tissue, transplant biopsies, and neonatal specimens are treated the same way: the incision is already closed or the infant cannot be recollected without harm.

A practical STAT queue

Invasive lower-respiratory specimens (bronchoalveolar lavage, protected specimen brush, bronchial washings) outrank expectorated sputum. They are obtained under sedation or intubation, they support quantitative thresholds used for ventilator-associated pneumonia, and they cannot be casually repeated. Instrument-positive blood-culture bottles are pulled when the continuous-monitoring system alarms; they are already growing and become a laboratory emergency even though the original venipuncture happened hours earlier.

Routine sputum, midstream urine, stool, genital swabs, and surface wound swabs are clinically important but sit later in the queue unless the requisition documents a true emergency such as suspected necrotizing fasciitis (treat that tissue as a sterile-site STAT). A working M-exam order is: (1) CSF and other sterile fluids, (2) tissues and deep aspirates, (3) invasive respiratory specimens, (4) flagging blood-culture bottles, (5) sputum, urine, stool, and swabs. When two STATs arrive together, the specimen that cannot be recollected wins. Do not hide a CSF behind a stack of outpatient urines—the exam writes that sequencing error on purpose.

Rejection criteria

Rejection is a documented patient-safety action, not a scolding of the collector. Notify the authorized provider, explain why a result would be misleading, and request recollection when the patient can still provide material. Unlabeled or mismatched identifiers are an automatic reject. Microbiology requires two unique identifiers that match the requisition; a room number, a first name, or a label from a different patient is not close enough. Leaked or cracked containers are rejected because the exterior contaminates staff and the interior is no longer a closed culture.

Prolonged delay is source-specific. Unpreserved urine held at room temperature longer than about 2 hours overgrows urethral flora and inflates CFU/mL. Stool for culture delayed without Cary-Blair loses Shigella. Anaerobic swabs held in ambient air kill the organisms the clinician wanted. Saliva submitted as sputum is a high-yield reject: numerous squamous epithelial cells and scant neutrophils mean oropharyngeal material that will grow viridans streptococci and saprophytic Neisseria, not the pneumonia pathogen. Quantity not sufficient (QNS) applies when volume cannot cover every ordered test—for example a few drops of CSF when Gram stain, culture, cryptococcal antigen, and multiplex PCR are all requested.

Specimens that are already nonviable

Formalin-fixed tissue is rejected for culture. Formalin kills bacteria, mycobacteria, and fungi; rinsing a histology block does not restore viability. Culture and histopathology must be split in the operating room. Dried swabs are rejected because organisms have already died in the fibers. Do not set up anaerobic culture on voided urine, sputum, stool, or vaginal swabs—those sites either harbor resident anaerobes or are incompatible with anaerobic interpretation. Stool culture from a patient hospitalized more than 3 days is typically cancelled in favor of Clostridioides difficile testing unless a special indication is documented. Duplicate same-source cultures within 24 hours may be rejected by policy. The exam answer is almost never “plate it anyway and add a footnote.”

Biosafety cabinet and PPE

Process respiratory specimens, tissues that may contain dimorphic fungi or mycobacteria, and any rule-out select agent inside a certified Class II biosafety cabinet. Standard PPE is gloves, a fluid-resistant laboratory coat or gown, and eye protection. Add a fit-tested N95 respirator or PAPR for aerosol-generating AFB procedures. Use aerosol-tight centrifuge cups, keep plate lids over agar as shields, and disinfect the cabinet with a mycobactericidal agent after AFB work.

The signature exam trap is processing AFB specimens in the same BSC session as routine bacterial cultures. Mycobacterium tuberculosis aerosols settle onto neighboring blood-agar and chocolate plates and later grow as false-positive TB cultures—patients are isolated and treated for laboratory contamination. Dedicate cabinet time to AFB, process one sediment at a time, keep caps on during vortexing, and never treat the TB hood as extra space for routine sputum plating.

Preparation methods

Concentration increases yield from large-volume, low-burden fluids. CSF and other sterile fluids are centrifuged; the pellet is used for Gram stain, culture, and often molecular testing. Do not discard the supernatant until every ordered assay is accounted for.

Digestion, decontamination, and TB cross-contamination

Digestion and decontamination are mycobacteriology steps for contaminated sites, especially sputum. N-acetyl-L-cysteine (NALC) liquefies mucus by breaking disulfide bonds; sodium hydroxide (NaOH) kills rapidly growing oral bacteria and fungi that would overgrow slow mycobacteria. Contact time is controlled (commonly about 15 minutes with NALC–2% NaOH in CDC-style procedures): too little NaOH leaves contaminants; too much kills mycobacteria. After neutralization and centrifugation, inoculate Lowenstein-Jensen, Middlebrook 7H11, and MGIT and prepare the AFB smear from the same pellet. Do not digest CSF or sterile-field tissue that has no competing flora.

Prevention of TB cross-contamination is a procedure of its own: unidirectional AFB workflow, single-use pipettes, negative processing controls when required, and no batch-opening of multiple sputa in a visible aerosol. Sterile technique on the open bench still matters for routine work—disposable or properly flamed loops, no glove prints on agar, lids held as shields.

Tissue work and molecular extraction

Tissue homogenization is organism-class specific. Bacterial abscess and tissue are often ground to release intracellular organisms. For fungi—especially Mucorales with fragile nonseptate hyphae—mince with a sterile scalpel instead of grinding. Grinding shears hyphae and can produce a false-negative mold culture, the second high-yield trap in this section.

DNA and RNA extraction for molecular microbiology must match the target. RNA viruses need RNase-free handling and reverse transcription; DNA assays still fail when blood, mucin, or residual formalin inhibit polymerase. Include extraction and inhibition controls, and keep molecular workflow unidirectional so amplicons from yesterday’s CSF PCR do not contaminate today’s extract. Do not perform nucleic-acid extraction in the AFB cabinet merely because it is a hood.

If you can apply that table without hesitation, you have the prioritization, rejection, and preparation cluster that I.B actually scores.