Air drying and heat fixing

For sputum smear microscopy, the slides should be air-dried as heating the slide while the smear is wet can result in bubbling of TB bacilli into the air.

Fixation makes the sputum stick to glass slide and preserves the shape of the bacilli. 

The procedure for air-drying and heat-fixing the slide is as follows:

• A smear prepared on a clean glass slide from mucopurulent portion of the specimen is air dried for 15-30 minutes on a rack (see Figure 1)

Figure 1: Rack for air-drying slides

• When dry, the smear facing upwards is fixed by heat from below. This can be achieved by passing the slide 2-3 times over the flame of a spirit lamp (as shown in Figures 2 and 3) for 3-4 seconds each time.

Figure 2: Fixing the Smear by Heat Fixation; Source: Laboratory Diagnosis by Sputum Smear Microscopy

Figure 3: Spirit lamp used to heat-fix smears

Important points to consider when fixing smears

• Heat fixing does not always kill Mycobacteria, exercise care when handling slides.
• Flame fixing may aerosolize bacilli from the smear.
• Overheating can damage the bacilli, burn the smear or break the slide.
• Insufficient heat or time can lead to smear washing off during staining steps.
• Heating for too short a period can result in a false-negative result because the TB bacilli will not be well preserved on the slide.

After the smears are fixed, they can be stained for examination or stored, or used in proficiency testing panel and quality control slides for staining.
 

Resources

Laboratory diagnosis by sputum smear microscopy

Method for Inactivating and Fixing Unstained Smear Preparations of Mycobacterium tuberculosis for Improved Laboratory Safety

Assessment