smear preparation procedures
should be prepared in manageable batches (maximum of 12 per
batch). Labelling of smears should be done at the bench for
incoming specimens using a permanent marker, eg. a diamond-point
stylus or wax pencil. Avoid touching the surface of the slides.
maximum chance of finding bacilli in unconcentrated specimens
is in the solid or most dense particles of the sputum and the
results of direct smear examination depend to a great extent
on the choice of these particles.
procedure for smear preparation is presented in Diagram 1 on
of smears on extra-pulmonary specimens
M. tuberculosis may infect almost any organ in the body,
the laboratory could receive a variety of extra-pulmonary specimens,
eg. body fluids, tissues, pus and urine. The benefit of microscopy
on these specimens is limited and it is recommended that extra-pulmonary
specimens be referred for culture.
smears should be avoided as the results can be misleading. Acid-fast
bacilli are frequently present in food and water and hence in
the stomach. There is no way of distinguishing such organisms
from tubercle bacilli on microscopy and positive smears must
be regarded with suspicion.
smears are almost useless. A negative result is meaningless
and it is best to conserve what little material there is for
and thick aspirates
Direct smears of
these specimens should be very thin. Thick smears tend to float
off the slide and even if they are retained, acid-fast bacilli
may be difficult to see after staining. Problems may arise if
a large amount of blood is present in the specimen since blood
may sometimes produce acid-fast artefacts.
fluid should be centrifuged and smears prepared from the sediment.
Again, these should be thin or they may float off the slide.
from cerebrospinal fluid are rarely positive and sediment from
concentrated specimens should rather be cultured. If a smear
is desired, two parallel marks, about 10mm long and 2mm apart,
should be made on a clean glass slide. A loopful of the sediment
is then spread between these marks and the smear allowed to
dry. Another loopful is then spread over the first. When this
is dry the process may be repeated, depending on how much sediment
is available. This procedure clearly marks the area to be searched
for acid-fast bacilli. It is desirable that the smears are examined
by two independent microscopists. Clots should be saved for
of centrifuged urine deposits are very unreliable and should
be avoided. Mycobacteria other than tubercle bacilli are sometimes
present in urine, either when it is voided or as a result of
poor collection techniques. The presence of acid-fast bacilli
in urine should be viewed with suspicion.