FDA Clears First Test To Quickly Distinguish MRSA And MSSA

There is good news today from the medical device section of the FDA, with the approval of the first test to quickly distinguish MRSA from MSSA (HT @EyeOnFDA). This new test, the KeyPath MRSA/MSSA Blood Culture Test, is said to differentiate between the two in about five hours after growth is detected in blood cultures.

So how much difference will this make in patient care and the MRSA epidemic? I suspect not much…

First, the test detects MRSA in blood cultures. While handy, most patients with MRSA do not have blood stream infections. Instead, they are colonized (just have it happily coexisting with them) or infected in wounds, sputum or, less commonly, in urine. Many people simply carry Staph, including MRSA, in their noses. This asymptomatic carriage and shedding of MRSA is what leads to outbreaks, and why many hospitals now screen high-risk patient admissions for the organism, or isolate such patients presumptively. So this test is unlikely to lead to much change in isolation practices. It might be quite useful, however, if it can rapidly detect MRSA in wounds.

If someone is seriously ill, most physicians will now empirically treat for MRSA, along with giving other broad-spectrum antibiotics, since the mortality of septic patients is quite high. Generally, after 24-48 hours, when culture results are initially available, an attempt is often made to narrow the spectrum to use more specific drugs. This reduces side effects and emerging antibiotic resistance.

Where the KeyPath MRSA/MSSA test reportedly has significant value is in its ability to do rapid susceptibility testing, important because there are major differences between many of the “community-acquired” MRSA and “hospital-acquired” strains. They report using a “Bacteriophage Amplification Technology,” which is well described by Drew Smith.

I will likely find this test of some use, as it will guide my therapy and lead me to narrow the spectrum of antibiotics I am prescribing up to a day earlier. I am a narrow-spectrum antibiotic kind of Infectious Disease physician. Some of my colleagues, whether because of their training or by having been burnt by experiences, tend to use very broad-spectrum drugs. This is especially likely to occur with more severely ill patients—even if there are no positive cultures—both because of genuine concern and because of fears of malpractice suits.

I don’t believe having this test will dramatically change practicing patterns, however. There is ample evidence that physicians don’t change their prescribing habits in response to data. In one widely cited study, 47% of docs made changes in response to susceptibility test results, but “but only 50% of the changes were considered appropriate.” In contrast, Berild, et. al. found 88% of their docs adjusted antibiotics, resulting in narrow spectrum use in 80% and a 23% cost savings. The findings of this 2006 Norwegian study were much more positive than others I have seen reported, or seen in my practice.

But the other factor arguing against this new rapid diagnostic test’s having great impact is the psychological need of the doc—a need that often seems to overcome reason. If a patient is better, many are loathe to change course—just in case, or “not wanting to argue with success.” Magical thinking, perhaps?

I can certainly see many uses for this new Bacteriophage Amplification Technique for antibiotic susceptibility testing. We can use all the help we can get in the fight against antibiotic resistance, and rapid diagnostics would be a big help. What kind of impact do you think this new KeyPath MRSA/MSSA test will have?