Will rapid tests ever fully replace cultures?
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Not in the foreseeable future.
No. Used rationally, rapid nonculture tests can be a powerful complement to cultivating organisms. However, they will not fully replace cultures for the foreseeable future. Having an organism in hand allows phenotypic testing — most notably of antimicrobial susceptibility — and strain typing. Cultures can confirm results of rapid tests. They can provide insights not available by certain rapid tests, such as detection of organisms not included in a particular diagnostic panel. They can give noncorroborative data that are important in shaping interpretation of rapid test results and in guiding clinical decision-making.
Collections of well-curated strains are also important research tools for understanding evolution, genomic epidemiology, transmission, ecology, emergence of antimicrobial resistance or other important phenotypes. At present, clinicians are well accustomed to interpreting culture results and considering issues such as colonization vs. disease, potential for contamination, etc., in their decision-making. In general, clinical decision-making with rapid nonculture tests is not yet at the same level of sophistication. Until clinicians use and interpret rapid tests more consistently in a Bayesian manner, their full potential will remain untapped, and cultures will serve as crucial arbiters of uncertainty and as correctives to misinterpretation.
Cornelius (Neil) J. Clancy, MD, is a professor of medicine and director of the extensively drug-resistant pathogen lab and mycology program at the University of Pittsburgh.
Most will be replaced.
There are a couple of key words in there. First, “ever” is too long. That’s not something that we can really say because there are a lot of things that, when I was just starting out a few decades ago, I just wouldn’t have imagined could ever happen. Second, “fully.” It’s hard to think of a case where there wouldn’t be some reasons sometime to do cultures, so that may be asking too much to say “fully,” but you could say “largely.” Will they replace most cultures?
We’ve already gone down this path with several different things that we used to culture. For example, Clostridium difficile cultures were available decades ago, but this is now mainly detected by PCR and immunoassay. Herpes simplex virus is another thing that, until not too long ago, we grew by culture, but it’s now almost all done by PCR. So, that’s clearly the direction things are heading, and I think the question worth asking is how far that’s going to go and how much it will replace cultures. My guess is that most routine cultures are going to be replaced by these tests.
Usually, when people are talking about rapid tests, what they mean are lateral flow, immunoassay tests, like we do for some of the home tests for COVID or have done for influenza or strep throat. Those are rapid, but there’s more to it than the rapid part there. These are tests that are usually done at point of care. They have the advantage that they can be deployed very cheaply, but they also have the disadvantage that they’re not culture. They typically have reduced sensitivity compared with nucleic acid tests. That category of rapid tests is good when you have a specific question like, “Do you have the flu?” However, if you have a UTI, there are dozens of organisms that are possibly there. Those tests are not really going to be appropriate because there’s no feasible way to include all the things that might be there. So, rapid tests won’t really replace those kinds of things.
There are some tests now — PCR or nucleic acid amplification tests — that are done for COVID-19. They give results in 15 minutes or so and they will almost certainly see increasing use relative to culture. And then there are other tests that principally can be done rapidly that are completely different in technology.
I would say in the short term, there’s still no alternative to culture because of the antimicrobial resistance piece of the puzzle. In the long term, who can say? Anything can happen. In the medium term, [speed and other factors] are going to push more and more things away from culture and toward molecular detection of one sort or another and that will have both gains and losses to clinical care overall.
Robin C. Colgrove, MD, FIDSA, is an attending physician in infectious diseases and medical director for diagnostics at Mount Auburn Hospital in Cambridge, Massachusetts, an assistant professor of medicine at Harvard Medical School, and chair of the Infectious Diseases Society of America’s diagnostics committee.
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