Transcript

My name is Dot Weir, RN, CWON, CWS. I'm a wound care nurse of many years. I practice currently in Saratoga Springs, New York at the Saratoga Hospital Center for Wound Healing and Hyperbaric Medicine.

Can you briefly discuss how a clinician can determine WHEN to culture a wound?

So many people think of culturing as diagnosing a wound and the diagnosis of wound Infection is really a clinical one. You look for your typical signs and symptoms, increased drainage, increased odor, erythema, pain at the site, purulent exudate, etc, just something that tells you that the body is reacting. It's based on those clinical signs and symptoms that, since you're going to make the decision to treat systemically, then we would do a wound culture in order to identify the organisms that are growing and what antibiotics might be appropriate to treat those organisms.

What information can clinicians expect to learn from a wound culture?

One of the first things, and this is part of ordering the culture, is you want to always order a gram stain. A gram stain is a technique that when they first get the culture they can they actually stain it in a couple of different ways, there’s a process it goes through. If it holds on to a purple stain, which is called the gram stain, then it identifies as it being a gram positive, if it holds on to the red stain then it's gram negative. 

What that first does is identify a particular bacteria that might be involved. For example, some gram positive might be Staphylococcus aureus. If you're treating for Staphylococcus aureus, then, if you already started somebody on a treatment for Staphylococcus aureus, then you may be on the right track. The importance of getting a gram stain, though, is that it will be ready within a day. And if you, depending on where the lab is, you can get this information sometimes in 3 or 4 hours. It gives you some early information. It'll say if there's white blood cells, so it'll tell if the body's reacting to whatever bacteria are there. But it gives you some early information that might help to initially guide treatment until you get the final report. 

Once you get the final report, and this in most cases for most labs takes 3 days, now there are some alternative types of ways to get cultures by sending them off for DNA sequencing, but we'll stick with the basics for this. But when you get the report from the Gram Stain after 3 days: it will confirm the Gram Stain. It'll identify what the organism was or organisms were that were obtained through the culture. And then it tells the antibiotics that it was tested against and then it'll usually give some kind of quantifying information. Now, if it's just a qualitative culture, it will only tell you the organism, the antibiotic tested, and the mean inhibitory concentration of that antibiotic. 

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It won't give you any sort of sense as the quantity of the of the bacteria that are there. There are semi-quantitative results. Semi-quantitative usually are either reported as 1 plus, 2 plus, 3 plus, or 4 plus, and that's the way they cultured out in the petri dishes. Or now they might have small growth, moderate growth, large growth, or at least some kind of quantitative information to let you know if it seems to be only a minimum amount of growth or a larger amount of growth. That's the kind of information that you get. And by identifying what antibiotics were tested against it in the lab, it will also give you the minimum inhibitory concentration. So most of the time you want to go for the lowest minimum inhibitory concentration for what antibiotic is going to work. 

Now, that may differ in some labs or in some hospitals, say an infectious disease doctor may say, “well, I know that has the lowest MIC, but this particular species in this hospital is better treated with this particular antibiotic.” But for basic information, you get the gram stain, you get the organisms that grew out, you get the the minimum inhibitory concentration of the list of antibiotics that they tested against, and then in a semi-quantitative, you will get a sense of how many are there. Lastly, if you get a true quantitative culture, you actually probably sent in tissue, and we'll get to how we obtain these samples in just a moment, but you will actually get a number. 

That number is usually a logarithmic number and it will say you know greater than 100,000 colony forming units per gram of tissue, for example, and that's what some people hold as the definition of infection, unless it's a strep and you treat as if it's just one. It’s a numbers game but when you get a quantitative culture, they will tell you an exact number and so it can be as low as say 10⁴ which would be 10,000 colony forming units. People take those numbers and that determines just really how densely populated that the specimen was with the particular bacteria that grew.

What is the best technique by which to perform a culture?

The best technique to perform the culture? I'm going to give you 2. If you're taking an exam or you're going to look at what is the gold standard, it would be a quantitative tissue biopsy, either using a punch biopsy or cutting out some tissue and sending that off for quantitative culturing, which is the logarithmic one I already described. That's still considered to be the gold standard, but I would tell you it's not the standard of care. Most people do not do tissue biopsies for microbiology. They do it for pathology, certainly. And the reason is because it's invasive, a lot of these are done outpatient. Many times it's a nurse that's taking the culture and many nurses are not able to cut tissue out of somebody’s wound. Most people use a swab to do the culture. 

Now here's where the rubber meets the road on this. It is imperative that we do a good swab culture, because it's just like with computers, garbage in, garbage out. If you do a bad swab culture, you're going to grow a lot more that is problematic for the wound and that could lead to overuse of antibiotics. So the best way to do it is first, clean the wound. If it's possible to have the wound debrided if there's necrotic tissue there because we don't want to culture necrotic tissue that would be the best thing to do. 

And then importantly clean the wound again because after a debridement you're going to leave some bits of tissue behind and it may be necrotic tissue that would alter the culture further. So clean, debride, clean, and if you use an antimicrobial cleanser, I would do a final rinse with some normal saline. Then, you take your culture swab and you press it into the tissue and then roll it around while pressing or depressing it down into the tissue. 

What you're trying to do is gather some fluid that these cells are bathed in and that will tell you if there are organisms there. So while it's not as perfect, it's not as ideal as a quantitative tissue biopsy, there's like a 70-75 % correlation between the 2 and some studies that have been done. And so a well done swab culture and that technique that I just described is called the Levine technique. And you should look that up if you're not familiar with it and read it again. But the Levine technique is the best way to get a culture that is clean. Oh, and the other thing too, is wherever you're taking the culture in this wound, don't go for dead tissue. You want to find the cleanest tissue possible in order to get this culture. And it will net you the best results. And again, there's a 70-75 % correlation in studies that have been done comparing it to a quantitative culture.

How can clinicians translate the findings of a culture report into actionable treatment steps?

To translate this information now that this culture is going to give you, it's really going to depend on the information that you're given. I'm going to lay this out in the a couple of different ways. If you did the culture with the intent to treat with antibiotics, the cultures these are going to tell you when antibiotic is the best, or it will confirm that perhaps the antibiotic that the patient is already taking is the right one, or it will show that that wasn't the right antibiotic that can be stopped and a new antibiotic can be ordered. 

Now, that's the intent of doing a culture, is to treat the patient. What happens sometimes is well-meaning clinicians will look at a wound or smell something and say, “gosh, I think this might have some bacteria” and they'll do a culture, they'll get an order for it, like a home nurse might call and say, “I think this wound smells bad” and they may get an order from a primary provider to go ahead and do a culture. And then the person who's managing that wound, the physician or the provider, will look at the wound and say “this is not infected” and make the decision not to treat. However, if there are signs and symptoms that are more covert, like a little more exudate, some odor that wasn't there before, the exudate looks cloudy. 

Our best option is to always first treat topically using a topical antimicrobial, an antimicrobial cleanser such as hypochlorous acid or whatever you might have, a topical antimicrobial such as a silver dressing, you know, whatever somebody has and try topical before we go to systemic. If somebody is not showing more overt signs of infections and some of the things that I mentioned at the beginning, then they probably don't need systemic antibiotics. We want to always be practicing antibiotic stewardship and not over treating. 

Hopefully, we've gotten a good culture. Some people may get a culture and if it's only 1 plus or just a small amount, they still may choose not to treat. It's a, decision that's made based on signs and symptoms of what we're looking at. That's what we're going to do with the culture results. But again, if we've cultured with the intent to treat, and I'd like people to think of it that way, culture only if you feel like you're going to treat this with antibiotics, then that information is going to give you what you need to know to treat with appropriate antibiotics. Otherwise, we should treat topically.