What smear, NAAT and culture each tell you
A smear (a stained film) shows structure, a NAAT (copies and detects a short genetic sequence) shows target sequence, and culture shows organisms are viable (alive and able to multiply).
Remember
A smear gives a quick look at structure, a NAAT shows that target sequence is present without proving the organism is alive, and culture is the method that shows viable growth and yields an isolate (a living pure sample).
Memory aid
a NAAT finds the footprint; culture proves that the animal was alive and gives you the animal.
A NAAT shows the target sequence is present, like a footprint, while culture shows that the organism was alive and supplies an isolate.
A smear (a stained film) is read by microscopy, which looks at the organisms themselves, so when enough organisms are present and the right stain is chosen it can give rapid information on their shape, their arrangement and how they take up a stain (a dye), within the hour.
A positive smear narrows the possibilities but does not name the species. A Ziehl-Neelsen smear (the stain used to find acid-fast cells) that shows red rods against a blue background means acid-fast bacilli are present: rod-shaped bacteria with waxy walls that hold on to the red dye while the dye is removed from other material. The smear does not separate the Mycobacterium tuberculosis complex from other acid-fast organisms, and it does not by itself show that the organisms are alive.
A nucleic acid amplification test (NAAT) copies a short stretch of an organism's genetic sequence until it can be detected. A positive result tells you the target sequence is there. Because it needs no growth, a NAAT can be quick, and for mycobacteria (bacteria with a wax-like outer layer, many of which grow slowly) it can report much sooner than culture. But dead cells carry genetic material that can be detected too, so a positive result does not by itself show that the organism is alive.
Culture gives organisms suitable conditions and checks whether they grow. Growth shows that they were alive and able to multiply, which is what viable means, and it leaves the laboratory with an isolate (a living pure sample of the organism). That isolate can be used for further tests such as susceptibility testing, which shows which drugs will work. The tests complement each other rather than being interchangeable.
Key facts
- Smear
- gives quick information on shape and staining behaviour, but a positive film narrows the possibilities without naming the species.
- Acid-fast smear
- red rods against a blue background mean acid-fast bacilli are present, but the smear does not by itself show that the organisms are alive.
- NAAT
- copies and detects a short genetic sequence, and for slow-growing mycobacteria it can be quicker than culture.
- NAAT limit
- a positive result shows that the target sequence is present, but genetic material can outlast the organism, so viability is not shown.
- Culture
- growth shows that the organisms were alive and gives an isolate for further tests such as drug susceptibility.
- Species and drugs
- naming the species needs culture or molecular tests (such as a NAAT), and testing which drugs will work needs culture or a molecular test that looks for resistance genes, not a smear.
Exam traps
- A positive NAAT does not show that the organisms are viable (alive and able to multiply), because the same genetic material can come from dead cells as well as living cells.
- A positive acid-fast smear (one that shows acid-fast bacilli) can neither confirm nor exclude tuberculosis, because it cannot tell the Mycobacterium tuberculosis complex from other acid-fast mycobacteria.
- For many mycobacteria, culture is usually slower than a NAAT, not faster, and culture is valued because it provides an isolate for further tests.
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