Deficiencies leading to false negative results include

the following:

Inadequate Sputum Collection

The patient is sometimes not told clearly enough what

constitutes a proper sputum specimen and how he should

produce one. It must be made clear to him that saliva and

nasopharyngeal discharge are unsuitable for examination.

Patients should be encouraged and given time to produce

bronchial sputum from the “depths of the chest”. If

repeated attempts have failed, tickling of the inner surface

of the epiglottis or trachea with a swab, or intratracheal

instillation of 5–10 mL of cool saline or sterile water may

provoke a vigorous cough with sputum. Other techniques

to stimulate the production of sputum, such as aerosol

induction, gastric aspiration, and bronchoscopy, require

more complex equipment or special skills.

If a patient discharges acid-fast bacilli in his sputum,

these are more likely to be found in a specimen produced

in the early morning than in one produced later in the day.

If early morning sputum in required, the patient should be

given a container and instructed to place in it the very first

sputum he produces in the morning, before breakfast and

before taking any medicaments.

Improper Storage of Sputum Specimens and Stained

Smears

Acid-fast bacilli may lose their acid-fastness as a result

of exposure of the specimen to direct sunlight, radiation

(e.g. ultraviolet light), excessive heat, or storage for more

than a week in hot and dry conditions.

If Ziehl-Neelsen stained smears have to be stored for

reexamination, the immersion oil must be washed from

the smears with xylol because the immersion oil removes

the stain from the acid-fast bacilli.

Fluorochrome stained smears will lose their

fluorescence with storage.