when

several slides are treated simultaneously in staining or

decolorization tanks. This can be avoided by processing

each slide separately, e.g. on a rack. Such racks are usually

made of wire and can be decontaminated easily by flaming.

Acid-fast bacilli may also be transferred accidentally

when the glass rod or dropper used for placing immersion

oil on the slide touches the surface of a positive slide

and rubs off some material. The same can happen when

blotting paper is used for drying several stained smear

consecutively. Therefore, the blotting paper should

not be used at all, or for no more than one slide. The oil

dropper should not touch the smear, and the oil should be

allowed to drip freely on to the slide. For the same reason,

the surface of the slide should not be rubbed with the

oil immersion objective. Before a new slide is examined,

the oil should be wiped off the lens with a piece of cotton

tissue or, even better, with special lens-cleaning paper.

When microscopy is used for the detection of acid-fast

bacilli, slides should never be used more than once.

False Negative Results

False negative results are commonly due to deficiencies

in the preparation of the smear, in staining, and in

Contd...

854 Concise Book of Medical Laboratory Technology: Methods and Interpretations

scanning. Adequate collection of the specimen and

subsequent “selection of sputum particles are essential

to the preparation of a smear and should receive special

attention.