other microorganisms, inorganic materials

and artifacts.

Food particles

To eliminate these, the patient should rinse their mouth

with pure water and clean their teeth (without using

tooth-paste or disinfectant) before producing the sputum

specimen. It is even better if the patient produced the

specimen before breakfast or on an empty stomach.

Precipitated stains

Though these are quite easy to differentiate from acid-fast

bacilli, they may hamper reading or occasionally mislead

an inexperienced microscopist. Precipitates can be

removed by filtration of staining solutions. However, it is

safer to use freshly prepared solutions, filled into carefully

cleaned bottles, rather than stale staining solutions.

Saprophytic acid-fast bacilli

These occur in soil and water, and may occasionally

get into the specimen or smear during processing. This

can be avoided by using distilled or boiled water from

scrupulously clean containers.

Mycobacterium kansasii or Nocardia species

These occasionally occur in specimens. When they cause

pulmonary disease, they are usually present in large

numbers.

Spores of Bacillus subtilis

These are very rare, mostly of ovoid shape, and larger than

tubercle bacilli.

Fibers and pollens

Fibers, including those of wood, cotton, filter paper and

bamboo, usually occur singly, most often in only one

microscopic field. The pollen of certain pine trees is seen

as short, coccoid rods occurring very rarely in specimens.

Scratches on the slide

Scratches may sometimes retain the red stain and confuse

beginners. They are usually seen in parallel rows, are

generally longer than acid-fast bacilli, and are undulated.

They can be identified easily, because they are found in a

deeper layer on the slide, below the smear disappearing

when the cells (e.g. leukocytes) in the smear get focused on.

Contamination through the Transfer of Bacilli from

One Smear to Another

It may happen that acid-fast bacilli are transferred

accidentally from a positive slide to a negative one,