Failure to Select Suitable Sputum Particles for Smear

Preparation

Tubercle bacilli are most likely to be found in little blobs

(“lentils”) of greenish-gray or yellowish matter of a thick,

creamy consistency. (Such blobs usually consist of dead

caseous tissue eliminated from a cavity in the lung). If the

sputum is not treated by a special concentration procedure

involving centrifugation, these blobs have to be carefully

separated from the rest of the sputum and transferred to a

slide. They can be seen more easily in the sputum against

a dark background.

Inadequate Preparation of Smear or Staining of Slides

False negative results may be obtained also when:

a. Too little material has been spread on the slide, so that

the smear is too thin;

b. The smear is too thick, so that sufficient light cannot

pass through it;

c. The slide has been over heated when fixing the smear;

d. The smear has not been sufficiently fixed and parts of

the material have been washed off;

e. The staining with carbol fuchsin was too short or was

overdone by boiling;

f. The counterstaining was too intensive, so that the acidfast bacilli have been obscured;

g. Staining and counterstaining times have not been

followed precisely.

Inadequate Examination of the Smear

If the scanning is done erratically or too briefly, too few

fields may be examined (Occasionally the examiner is

unable to distinguish the red-stained acid-fast bacilli

because of color blindness or other visual disturbances).

Other Reasons for False Results

Administrative errors

Such errors may include:

a. Misidentification of patients, misspelling of names, or

confusion of names or of codes numbers of specimens

and slides;

b. Mistakes in labeling containers;

c. False recording of reporting.