sputum smear microscopy
Sputum smear microscopy is one of the oldest and most practical TB tests: a sample of coughed-up phlegm is smeared on a glass slide, stained, and examined under a microscope to look for acid-fast bacilli. It is the diagnostic equivalent of a quick, inexpensive scout sent ahead to see whether the enemy is present in large numbers.
Its great strengths are speed, low cost and minimal equipment, which is why it has long been a frontline tool in places where TB is common. A positive smear also flags the most infectious patients, because the bacteria only show up under the microscope when they are present in high numbers — roughly thousands per millilitre of sputum.
Its main limitation is sensitivity: people with smaller bacterial loads, including many children and people with HIV, can have active TB yet a negative smear. The test also cannot tell TB apart from non-tuberculous mycobacteria, nor detect drug resistance. For these reasons, smear microscopy is increasingly paired with or replaced by molecular tests and confirmed by culture.
In a busy TB clinic, three early-morning sputum samples are smeared and stained; finding acid-fast bacilli on the first slide allows same-day isolation and treatment decisions.
A rapid, low-cost first look for the TB bacterium.
Smear results are often reported by a graded scale (for example 1+ to 3+) that roughly reflects how many bacilli are present and thus how infectious the patient may be.