MIC - infectious disease

[indytravl]

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I'd like some help understanding how to use the MIC reports from microbiology. is there anyone, esp ID, out there who could put it into basics please? I understand it's the min inhib concentration but how to use the values clinically? Is it the smaller the number behind the < that indicates the best antibioitic (the drug the bug's most sensitive to)?

thanks for the help ahead of time!

 

[Critical Mass]

I'm a microbiology med tech, so maybe I can shed some light. "Less than" a particular value is usually indicative of a sensitive result, but it should also have an interpretation listed if there are CLSI guidelines available for the organism and antibiotic. The main thing is to ensure that tissue levels of your drug can be accomplished at the provided concentration for the infected site if you are using an MIC. DO NOT use the susceptibility report as the sole means of determining a course of therapy. One drug may read "<1" and another "<2," but that doesn't mean that one is necessarily better. It just means that 1 and 2 were the lowest levels tested, respectively. Other than that, a lower reported MIC might exist for a given drug, but what if the drug costs a fortune or should be reserved for already multiply resistant bugs? Drug cost is usually listed on the report as well for inpatient labs. Just keep in mind that different labs use different systems to test, so reports will likely vary between facilities.

Susceptibility patterns are usually tested on automated systems that utilize different concentrations for different drugs. Sometimes supposed sterile body regions are harder to reach. Urine is usually an easy concentrator of drug.

Most of the time, docs use guidelines based on the particular organism and/or type of infection. There are cheat sheets for this. The susceptibility report is typically used when the infection fails to clear or if it reflects a result contrary to current therapy utilized.

Don't ask them to test a drug that is inappropriate for a particular organism. They won't do it.

 

[indytravl]

Thanks for responding.

It's unclear how the MIC report on a culture & sensitivity is used clinically. What would be "an interpretation listed if there are CLSI guidelines available for the organism and antibiotic?" I didn't note anything like that on the paperwork. And can you let me know some resources for the "cheat sheets" you noted?

these have all been on urine cultures & don't remember the precise details but one was something like:
>100,000 citrobacter
levofloxacin <2 susceptible (empirically started on it after + u/a)
>100,000 enterococcus
MIC
nitrofurantion 32 intermediate
levafloxacin <8 resistant
vancomycin <0.5 susceptible
tetracycline resistant
penicilling resistant

Pt cont'd to be febrile (99.7-102F) in period between u/a & results of c & s.

Further advice/thoughts?

thanks

 

[Critical Mass]

The cheat sheet I've used is a pocket Sanford guide:

http://www.sanfordguide.com/

It's a little booklet that fits in the pocket of your white coat, but these days it's in PDA form too. CLSI (Clinical and Laboratory Standards Institute) is the new name for NCLS. They decide whether a given MIC suggest sensitivity or resistance. In all of the labs I've worked in, our printout reads "R," "S," or "I" after the MIC (resistant, susceptible, or intermediate). It appears that your report actually spells them out rather than using R/S/I so that is cool too.

Looking at your report, I assume that 2 bugs grew out of the same urine specimen. The Citrobacter should always be treated as if it is resistant to ampicillin, and inducible beta lactamases should be thought of if you leave the pt on a beta lactam. In this case, Levo was used, so that shouldn't be an issue. (Nonetheless I'd reculture if the pt doesn't improve after a while.) The other bug, Enterococcus, doesn't make much sense to me. <8 for levo is not something that looks right as being considered resistant, but I cringe when I see fluoroquinolone resistance in Enterococcus out of a urine.

I guess that the take-home message might be that if you see "resistant" or "R," the drug is no good. Sensitive (S) usually suggests that it is okay, but there are times when the MIC for one sensitive drug is better than another (though these will be specific MIC's, not "less than").

I hate to get into too much detail because after all, I am a lowly MS 1. The best info I can give you is as a lab guy, not an ID doc. 🙁