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)

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.

How it works

The verdict depends on two low-power-field counts:

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.

Example and notes

A slide showing 4 squamous epithelial cells and 30 polymorphs per low-power field is acceptable — few mouth cells and a good inflammatory response — so it proceeds to culture. A slide with 25 squamous cells is rejected and a repeat early-morning deep-cough sample is requested. Average your counts over several fields, and remember these criteria apply to routine expectorated sputum, not to lavage, mycobacterial, or fungal specimens.