What are the Murray-Washington criteria?

They grade a sputum sample by the cells seen at low power. A good lower-respiratory sample has fewer than 10 squamous epithelial cells and more than 25 polymorphs per low-power field. Many squames indicate oropharyngeal contamination by saliva, which makes culture results unreliable.

Why reject a sample with many squamous cells?

Squamous epithelial cells come from the mouth, so their presence in large numbers means the specimen is mostly saliva rather than secretions from the lower airway. Culturing such a sample grows mouth flora and can mislead treatment, so a repeat early-morning deep-cough sample is requested instead.

What if there are few squames but few polymorphs too?

A sample with under 10 squames but fewer than 25 polymorphs is not clearly contaminated but shows little inflammatory response. The tool flags it as borderline or low-yield; it may still be processed in immunosuppressed patients or where a specific pathogen is sought, at the laboratory's discretion.

Does this apply to all respiratory samples?

No. The criteria are validated for expectorated sputum for routine bacterial culture. They are not applied to bronchoalveolar lavage, samples for mycobacteria or fungi, or specimens from intubated patients, where different acceptance rules and quantitative cultures are used.

What counts as a low-power field?

A low-power field is the area seen down the microscope at the 10x objective (roughly 100x total magnification). Counts are averaged across several representative fields rather than taken from a single field, to smooth out uneven distribution on the slide.

Sputum Gram Stain Quality Checker (Murray-Washington Criteria)

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Before a sputum sample is cultured, a Gram stain is screened at low power to check it actually came from the lower airway rather than the mouth. This tool applies the Murray-Washington criteria to your cell counts and tells you whether the specimen is acceptable or should be rejected as salivary contamination.

The Murray-Washington criteria at a glance

Squamous epithelial cells (SEC) — from the mouth (saliva)
Polymorphs (PMNs)               — inflammatory cells from the airway

Acceptable (lower respiratory): SEC < 10  AND  PMN > 25
Reject (salivary contamination): SEC >= 10
Borderline / low yield:          SEC < 10  AND  PMN <= 25

Plenty of squamous cells means saliva, so the sample is rejected regardless of the polymorph count. A clean sample with a brisk inflammatory response is the ideal specimen for routine bacterial culture.

Why this screening step matters

Culturing a salivary sample is worse than not culturing at all. A specimen dominated by oropharyngeal flora will grow a garden of commensals — viridans streptococci, Neisseria species, coagulase-negative staphylococci — none of which reflect what is happening in the lower respiratory tract. A clinician acting on that culture result risks treating the wrong organism or, worse, escalating to broad-spectrum agents unnecessarily.

The Gram stain screen catches poor samples before they reach the incubator, saving bench time and protecting the quality of clinical decisions.

Example reads

SEC/LPF	PMN/LPF	Verdict	Action
4	32	Acceptable	Proceed to culture and sensitivity
2	18	Borderline	Lab discretion; process if patient is immunosuppressed
25	40	Reject	Request repeat early-morning deep-cough sample
12	5	Reject	Heavily salivary; reject regardless of PMN count

Practical notes on counting technique

Count at 100× total magnification (10× objective × 10× eyepiece), also described as a low-power field. Counts at 400× are sometimes used but the thresholds are not validated there.
Average across at least five fields chosen from different areas of the slide. Sputum is heterogeneous and a single field can be misleading.
Count only intact cells. Lysed squames and bare nuclei should not be tallied as full SECs.
Record the predominant organism while you are screening. If the specimen passes and a clear predominant morphotype is visible (e.g., Gram-positive diplococci), note it — it should correlate with the eventual culture growth and provides an early clinical steer.

When these criteria do not apply

The Murray-Washington criteria are validated for routine expectorated sputum for bacterial culture in a patient capable of producing a genuine deep-cough sample. They should not be applied to:

Bronchoalveolar lavage (BAL), bronchial washings, or protected-specimen brush samples — these have their own quantitative culture thresholds.
Induced sputum (hypertonic saline induction changes the cellular picture).
Specimens submitted for mycobacteria (AFB smear/culture) or fungi — cell counts are irrelevant there; the concern is specimen volume and processing method.
Mechanically ventilated patients whose secretions are suctioned from the lower airway directly — these samples bypass the oropharynx and are inherently more reliable regardless of SEC count.