“The Sputum Factory”

It is the peak of the (Southern Hemisphere) influenza season here in New Zealand. Influenza has come early this year, and the season has been relatively busy to date, with a fairly even mix of Influenza A&B.

And it is not just influenza, there is plenty of circulating RSV, rhinoviruses and all those other less well known respiratory viruses that we know best by their appearance on extended respiratory viral panel menus.

However it is not just GPs and Emergency departments that feel the effects of the wave of respiratory viruses during the winter season. Microbiology departments get a surge in sputum samples arriving at the laboratory. Recently it has felt like my laboratory is simply a sputum processing factory!

And this is because any microorganism which infects the respiratory epithelial cells (bacteria or viruses) will inflame the respiratory epithelial cells and increase the production of “purulent” sputum. It doesn’t matter whether it is a bacterium or virus. Can the sputum colour discriminate between the two?? Only in the textbooks…

Sputum for bacterial culture is one of my least favourite microbiology samples! This is not just because of its appearance, but more because it suffers from appalling sensitivity and specificity when diagnosing bacterial pathogens, even when a pre-screening Gram stain is performed, as many labs do nowadays.

If sputum culture was subject to to FDA approval as a “diagnostic assay”, it wouldn’t have a prayer…

During the winter season when respiratory viruses abound, the prevalence of bacterial infection in the tested population will be relatively lower, with a consequent further deterioration in positive predictive value.

There are many guidelines that show the extremely limited value of sputum culture, particularly from the community setting, for the management of non-specific cough symptoms, acute bronchitis, and COPD. The most common clinical details we receive on sputum samples are things like “Cough”, “cough with purulent sputum”, “COPD”, or simply nothing at all. Yet we still accept these samples without question.

As a profession, sometimes I think we are too soft…

My lab is planning to introduce restrictions on what sputum samples are acceptable from the community setting, according to the published guidelines, so hopefully by next winter, we will be a haven of tranquility as opposed to a sputum processing factory.


Apologies for the picture, but it had to be done!


7 thoughts on ““The Sputum Factory”

  1. Well written on realities of culturing and observation of sputum specimens, for any learned interpretation of sputum work up is difficult how much of our reporting will help the clinicians as it happens many times clinician go by their own experience of dealing the matters ,
    Dr.T.V.Rao MD

  2. I have not come across a pre-culture screening Gram stain on sputum previously and was of the understanding that it was an old test no longer commonly performed. Do you do them in your lab? What is the benefit?

    1. Definitely not an old test! Most labs in New Zealand will do a pre-culture Gram stain, looking for neutrophils and epithelial cells and then set acceptance criteria as to what sputa merit culture based on these results. We culture approximately half of the sputa that we receive into the lab.

  3. Bravo Michael! Sputum has been the “sacred cow” of specimens that some physicians refuse to give-up. I started my clinical microbiology career in 1971 and the sputum specimen has always been the 2nd most frequent specimen next to urine. So many times the physicians, looking for something to treat, subsequently treat an “innocent bystander” microbe cultured from a sputum.

  4. A bunch of new fashion request coming in our lab recently, “? atypical pneumonia” . either someone provided wrong information or docs create a new way to ask lab “just do it!”. 😂😂😂

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