Heat fixed but unstained slide smears are infectious to laboratory staff

In a clinical microbiology laboratory, heat fixed slide smears are commonly transported from one place to another for staining with different stains and also for onsite proficiency testing of laboratory technicians for accreditation of the laboratories. These smears are frequently handled without gloves by the staff in developing countries. Therefore, this study was conducted to check the survivability of tubercle bacilli on smears after physical and chemical treatments.

A total of 196 AFB positive smears were analyzed. Of these, 116 were stained with Ziehl Neelsen (ZN), 60 with cold Kinyoun and 10 were unstained but heat fixed and 10 were neither stained nor heat fixed. The last 20 smears served as controls. The ZN and Kinyoun stained smears were 0-1.5-year-old and stored at room temperature in slide boxes, while control smears were freshly prepared. All smears were prepared from sputum samples positive for acid fast bacilli. All four sets were subjected to slide culture to see if mycobacteria could survive and grow in any. For slide culture, a new and safe device was used, which is designed for three in one purpose: cell cultivation, direct observation of the growth under microscope and cell harvesting inside the closed tube. The slide smears were directly dipped into this tube that contained liquid culture medium. The tubes were incubated at 37 degreeC for four weeks. The growth, if any, was confirmed by MPT-64 rapid test and subculture on LJ slants.

No growth was observed in ZN and Kinyoun stained slide smears. However, significant growth was observed in both control sets; the unstained non heat fixed as well as heat fixed slide smears.

The results of our study indicate that tubercle bacilli remain viable even after heat fixation and carry risk of infection by contact However, stained smears are safe for handling and storage.