Microscopy in TB Diagnosis

Microscopy is a TB diagnostic technology that utilizes the acid-fastness property of Mycobacterium tuberculosis to visualize it under a microscope. Results of sputum smear microscopy can either be smear-negative, or smear-positive (with various grades). 

Advantages:

• It is currently the most accessible and cheapest TB diagnostic test available under National TB Elimination Programme (NTEP) in India.
• It has the shortest turnaround time for diagnosis.
• It has high specificity. 

Limitations:

• Low sensitivity. It becomes positive only when more than 5000 bacilli/ml of sample are present. Hence, cases would be missed in early disease, or when an inappropriate biological specimen is provided, where bacterial load in sputum is less.
• It is unable to differentiate between M. tuberculosis and Non-tuberculous Mycobacteria (NTM). This is predominantly an issue in geographies with lower burden.

There are two types of microscopies used in NTEP: Ziehl-Neelsen (ZN) Microscopy and Fluorescence Microscopy (FM). These vary in the type of stain and microscope used. FM is newer of the two types and is currently recommended for use over ZN.

Resources

• WHO Policy Statement - Fluoresence Light-emitted Microscope for the Diagnosis of TB, 2010.