Medical expert of the article
New publications
Bacterioscopic analysis of sputum
Last reviewed: 04.07.2025
All iLive content is medically reviewed or fact checked to ensure as much factual accuracy as possible.
We have strict sourcing guidelines and only link to reputable media sites, academic research institutions and, whenever possible, medically peer reviewed studies. Note that the numbers in parentheses ([1], [2], etc.) are clickable links to these studies.
If you feel that any of our content is inaccurate, out-of-date, or otherwise questionable, please select it and press Ctrl + Enter.
Timely identification of the infectious agent is very important for the correct choice of antibacterial drug in order to prevent the development of bacterial resistance when an antibiotic is prescribed empirically. Gram staining is the most common method of staining all types of material obtained from a patient (sputum, bronchoalveolar lavage, etc.) for rapid and approximate identification of the infectious agent.
A preliminary assessment of a possible etiologic agent is made using bacterioscopy of a Gram-stained sputum smear. A Gram-stained sputum smear is examined before it is inoculated onto nutrient media also to assess its suitability for cultivation and to identify the probable pathogen. Sputum is considered suitable if more than 25 leukocytes and less than 10 epithelial cells are found in the Gram-stained smear at low magnification under a microscope. Signs of a high-quality sputum sample that can be used for cultivation include a predominance of leukocytes over epithelial cells, as well as the presence of bacteria of one species that are located inside or around the leukocytes. Gram-positive bacteria in the preparation are dark blue, and gram-negative bacteria are pink. The causative agents of atypical pneumonia (mycoplasma, legionella, rickettsia and chlamydia) are not stained by Gram, so serological methods are mainly used to detect them.
Sputum smear staining according to Ziehl-Neelsen is used to identify acid-fast bacilli, primarily tuberculosis mycobacteria. The preparation is prepared from purulent particles of sputum, which are selected from 4-6 different places. The selected particles are carefully ground between 2 glass slides until a homogeneous mass is obtained, dried in the air, and fixed over a burner flame. Tuberculosis mycobacteria are stained red, all other elements of sputum and bacteria are stained blue. Tuberculosis mycobacteria look like thin, slightly curved rods of varying length, with thickenings at the ends or in the middle, located in groups and singly. Detection of tuberculosis mycobacteria is the most reliable sign of tuberculous pulmonary disease. The Ziehl-Neelsen smear staining method for active forms of pulmonary tuberculosis has a sensitivity of 50% and a specificity of 80-85%.
[