13.2 Sputum Sample Collection: Spontaneous and Induced Specimens

13.2 Sputum Sample Collection: Spontaneous and Induced Specimens

Sputum sample collection is item 5 in Domain II of the PFT Detailed Content Outline, listed under Select Test Protocols and Equipment, Perform the Procedure, and Evaluate Validity of Result. Candidates routinely skip it because it does not produce a number on a PFT report. It is nonetheless a technologist procedure with a real safety profile — induction is an aerosol-generating procedure that can provoke bronchospasm — and specimen validity is a genuine analytical judgment.

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Indications and Method Selection

Select spontaneous expectoration when the patient produces sputum readily. Select induction when the patient has a dry or minimally productive cough, when a standardized specimen is required for cell counts, or when a spontaneous sample has already proven to be saliva.

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Spontaneous (Expectorated) Collection

1. Collect an early-morning specimen when possible; overnight pooling improves yield, particularly for mycobacteria and cytology.
2. Have the patient rinse the mouth with plain water — not mouthwash, which is antibacterial and will suppress culture growth. Remove dentures.
3. Instruct several deep breaths, then a deep cough from the chest, and expectoration directly into a sterile, wide-mouth, leak-proof container. Coach explicitly: "This should come from deep in your chest, not from your throat."
4. Target volume is small — 1 to 5 mL is sufficient; volume is not a quality indicator.
5. Label at the bedside with patient identifiers, date, time, and collection method (spontaneous versus induced), and transport promptly. If a delay is unavoidable, refrigerate at 2–8 °C, except for specimens intended for Neisseria or certain fastidious organisms, and never freeze a specimen intended for cytology without laboratory instruction.

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Sputum Induction with Hypertonic Saline

Hypertonic saline draws water osmotically into the airway lumen, thinning and mobilizing secretions while also stimulating cough. It is effective and it is provocative — the same osmotic stimulus is used deliberately as an indirect bronchial challenge.

Equipment

• Ultrasonic nebulizer with a high output rate (commonly 1–3 mL/min); a standard jet nebulizer produces too little aerosol for reliable induction.
• Sterile hypertonic saline, usually 3%, with protocols escalating through 4% and 5% and, in some, to 7%.
• Spirometer for FEV$_1$ monitoring, pulse oximeter, short-acting bronchodilator, sterile collection containers, and emergency equipment.

Protocol

1. Screen and consent. Confirm no relevant contraindication and explain the procedure and the salty taste.
2. Baseline spirometry. Record FEV$_1$. Induction is generally avoided when baseline FEV$_1$ is below 1.0 L or below 60% predicted without direct physician supervision.
3. Pretreat with a short-acting bronchodilator (commonly albuterol 200 µg by MDI with a spacer, or 2.5 mg nebulized) and wait 10–15 minutes.
4. Repeat spirometry. This post-bronchodilator FEV$_1$ becomes the reference baseline against which all subsequent falls are measured.
5. Nebulize the hypertonic saline in timed periods — typically 5-minute periods to a total of about 15–20 minutes.
6. Between each period: have the patient rinse the mouth with water, blow the nose, and cough deeply into the container; then repeat FEV$_1$.
7. Terminate immediately for a fall in FEV$_1$ of $\ge$ 20% from the post-bronchodilator baseline, for wheeze, chest tightness, or significant dyspnea, or at the patient's request. Some protocols step down rather than stop for a 10–20% fall; follow the laboratory's written protocol.
8. Post-procedure: administer bronchodilator if any fall occurred and observe until FEV$_1$ returns to within 10% of baseline.

Infection Control — This Is an Aerosol-Generating Procedure

Induction generates a high-volume respirable aerosol from the lower airway, which is exactly why it works and exactly why it is hazardous. Perform induction in an airborne infection isolation room (negative pressure, 6–12 air changes per hour, exhausted outdoors or HEPA-filtered) or a dedicated, well-ventilated booth. Staff wear a fit-tested N95 or PAPR, eye protection, gown, and gloves. Observe the room clearance time before the next occupant enters — roughly 69 minutes at 6 ACH and 35 minutes at 12 ACH for 99.9% removal.

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Evaluating Specimen Validity

A specimen that is mostly saliva will grow oropharyngeal flora and mislead treatment. Adequacy is judged microscopically, not by appearance or volume.

For bacteriology (Gram stain screening): an acceptable lower respiratory specimen shows more than 25 neutrophils and fewer than 10 squamous epithelial cells per low-power field. Squamous epithelial cells come from the mouth, so a high count means the specimen is saliva and most laboratories will reject it and request recollection.

• Important exception: this screen is not applied to specimens for mycobacteria, fungi, Legionella, or Pneumocystis, where a paucicellular specimen may still be diagnostic.
• Neutropenic patients may legitimately produce a specimen with few neutrophils; interpret in context.

For induced cell-count studies, the additional validity criteria are squamous contamination below about 20% of total cells and cell viability above 50% by trypan blue exclusion. Processing should occur within about 2 hours.

Other quality checks: reject a specimen that is unlabeled, that arrives in a leaking or non-sterile container, that was collected after mouthwash rinsing, or that is grossly clear and watery when a purulent specimen was expected. Always document the collection method — an induced specimen is diluted by saline, so cell concentrations from an induced sample cannot be compared directly with those from a spontaneous one.