Sputum smear, sputum culture, drug sensitivity test, how to check and test lower respiratory tract infection?

Author: Lu Binghuai, Chief Physician of China-Japan Friendship Hospital

Reviewer: Xiao Dan, Professor of China-Japan Friendship Hospital

For respiratory infections, the closer the specimens collected for laboratory examination are to the pathogen, the greater the chance of identifying the pathogen.

For example, throat swabs mainly reflect the condition of the upper respiratory tract. When we suspect infection with influenza A virus, influenza B virus, etc., we can take throat swab tests.

However, for infections of the lower respiratory tract, such as lung infections, bronchoalveolar lavage fluid (BALF) should be collected. However, the collection of BALF may be invasive and not suitable for outpatients or patients at risk. Deep expectoration may be used instead.

1. What should I pay attention to when collecting sputum samples?

In essence, sputum is not a very good sample because the coughing process of sputum samples is interfered by the normal flora of the oropharynx and may contain a lot of normal flora, which may confuse the real pathogens.

In order to avoid this confusion, it is generally necessary to rinse your mouth and brush your teeth before collecting sputum. Through this operation, the interference with normal flora can be minimized as much as possible.

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Then we need to cough up a mouthful of phlegm. The phlegm coughed up from deep inside can better reflect the situation of pathogens in the lower respiratory tract. If we just cough lightly, most of the phlegm coughed up comes from the upper respiratory tract, and there are no real pathogens in the lower respiratory tract. Sometimes the patient's ability to cough up phlegm is relatively weak, and relatives or caregivers need help to gently tap on the back to make it easier for the patient to cough up phlegm deep in the respiratory tract.

2. How are sputum smear, sputum culture and drug sensitivity test done?

Sputum smear:

After the sputum specimen is collected, a simple smear is made on the sample, followed by Gram staining or acid-fast staining. After Gram staining and magnification 1000 times under a microscope, we can see whether there are white blood cells and pathogens. Acid-fast staining can see Mycobacterium tuberculosis and non-tuberculous mycobacteria.

Simply put, if there are very few white blood cells in the sample, it means that it may not actually come from the lower respiratory tract infection site; if a large number of epithelial cells are seen, it indicates that the sample comes from the upper respiratory tract or oral cavity.

In general, a coughed-up sputum sample with more than 25 white blood cells/LP and less than 10 epithelial cells/LP can often be considered a qualified sputum sample, and the next step will be taken to identify the pathogens in it.

Sputum culture:

Laboratory personnel will inoculate sputum samples onto different culture media, allowing the pathogenic microorganisms in them to grow into visible colonies under suitable conditions.

Some bacteria grow relatively fast, such as Streptococcus pneumoniae, Haemophilus influenzae, Klebsiella pneumoniae, Pseudomonas aeruginosa, Acinetobacter baumannii and other common bacteria, most of which can grow within 24 hours. If multiple bacteria grow at the same time, it is necessary to continue to isolate individual colonies for the next step of inspection. The laboratory identifies these grown colonies, such as using biochemical methods or mass spectrometry methods, to determine which type of bacteria these pathogens are. Then a drug sensitivity test is performed to detect which type of antimicrobial drugs these bacteria are sensitive to, which can be used for the next step of treatment. The whole process takes about 3-5 days. For some rapidly growing bacteria, the inspection report can be obtained in 2-4 days.

Some pathogens grow slowly, such as Aspergillus fumigatus, which may take 2-5 days to grow, while Mycobacterium tuberculosis and Mycobacterium avium may take 20-40 days to grow. Only after seeing the colonies can it be identified and the drug sensitivity test can be carried out.

This is why the time for us to receive culture identification and drug sensitivity test reports is inconsistent when we do different tests.

Drug sensitivity test:

The drug sensitivity test is that we culture a bacterium and believe it to be a pathogenic microorganism, and we need to use in vitro tests to see which antibiotics this bacterium or fungus is sensitive to.

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Of course, not all detected pathogenic microorganisms need to undergo drug sensitivity testing. Because some bacteria are generally sensitive to many antibiotics, we do not need to do this kind of drug sensitivity test specifically. There is no standard method for drug sensitivity testing of some bacteria, or the conditions are too harsh for the laboratory to perform. These bacteria will not provide drug sensitivity reports, only identification results. Clinicians can choose antimicrobial drugs based on guidelines or expert consensus.

In clinical practice, collecting sputum specimens for smear, sputum culture and drug sensitivity test is the most common method for diagnosing lung infection etiology in respiratory medicine. Correctly collecting sputum samples, actively conducting relevant examinations, and rationally selecting appropriate drugs are key measures to promote the cure of the disease as soon as possible.