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Colony-forming Units (CFU) in Urine Culture (Nursing)

by Rhonda Lawes

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    00:00 Now, you'll see the things about colony-forming units. So I wanted to make sure you and I talked about it. What are colony-forming units? Well, in the urine culture, the number of colony-forming units per mL is an estimate of the number of bacteria in the sample. Okay, cool, that matters, right, because I'm going to have a harder time fighting off an infection that has a high number of colony-forming units versus one that has a lower number of colony-forming units. So we've established that colony-forming units help us estimate how much bacteria there is. Now, we have an actual definition of what infection is or UTI. So, lean in and take a look at that chart. Colony-forming units are on the left-hand side. So, greater than 10 to the 3rd power of the colony-forming units per mL in the urine culture indicates infection. Okay, let's start there. I'm going to give you some other tips after that, but let's just start there.

    01:02 Greater than 10 to the 3rd power which is, right, 1,000 CFUs per mL that means we have infection. So, if it's less than that, can the patient have a UTI? The answer is yes. Now, I'm not trying to confuse you, but what do we always talk about with lab work? You have to compare the number to the actual patient. So the patient may have a UTI if that CFU is less than 10 to the 3rd power. What has to be present though is symptoms and urinary leukocytes. So let's move over to the right-hand side of the column and talk about leukocytes. Now, when that's done on a microscopic exam, if you see greater than or equal to 10 leukocytes in the urinalysis, we can diagnose a UTI if they've got those symptoms. Remember this shouldn't be the sole criteria used for diagnosis but you're going to take the numbers that you have, compare them with the patient's symptoms. So, as a general rule, I want you to have greater than 10 to the 3rd power CFUs; however, if the patient has symptoms and greater than or equal to 10 leukocytes, we can still agree the patient likely has a UTI. I've organized this table for you, but I want to start in the middle. Now, I know that's not normally where we start but I want to on this one. Hang with me, my rationale make sense as we're done. So the middle part of the table says evidence of bacteriuria colony-forming units per mL. Okay, so it goes from 10 to the 3rd, 10 to the 4th, 10 to the 5th. So, we see that each diagnosis has a higher number of colony-forming units. Now, let's look at urine collection. The 1st one is midstream. The 2nd one is midstream, okay. The 3rd one gets way complicated. I'll come back to that in just a minute. So, let's start with 10 to the 3rd power. We're already familiar with that number.

    03:00 That's for an acute uncomplicated cystitis in women. So, we get a midstream urine collection, 10 to the 3rd power of colony-forming units. We know that this is typical of an acute uncomplicated cystitis in women. What is cystitis? Underline that word for me, -itis means inflammation. Oh, in this case, you're referring to the bladder. So, cystitis means inflammation of the bladder, just like cystoscopy means a scope looking at the bladder. So, cyst- means bladder. 10 to the 3rd power midstream urine collection, we're got an acute uncomplicated cystitis in women. Now, 10 to the 4th power we're taking it up a notch. We have more bacteria in the sample. So, knowing what we know about the urinary tract, urethra to the bladder, the ureters up to the kidney, okay if we have more bacteria that means it is traveling outside of the bladder. That's an evidence of acute uncomplicated pyelonephritis. Now, that's traveled up to the kidneys and these patients feel really bad. No one feels great with a UTI but when it progresses to a kidney infection, your patient is really going to feel bad. So, we've got the difference between cystitis bladder infection and an uncomplicated pyelonephritis is 10 to the 3 versus 10 to the 4th power CFUs. Now, let's breakdown that one at the bottom. What seems really ironic is it's asymptomatic bacteriuria. So, do they have bacteria in their urine? Oh yeah, look, 10 to the 5th power. This is the highest one that we have on the chart, but asymptomatic means the patient doesn't have symptoms. So, yes they have bacteria, but no they don't have symptoms.

    04:57 A- means without. So, asymptomatic bacteriuria has the highest bacterial CFU count. So, let's look at what's going on here. Well, this is the one that had a little bit more complex urine collection. For women, because of their anatomy, in order to be diagnosed with asymptomatic bacteriuria, they have to have 2 consecutive midstream urine samples. Okay, well, it's still midstream urine but in order to be diagnosed asymptomatic they have to have that higher CFU and we have to do 2 consecutive samples. In men, because of their anatomy, they get off a lot easier. They just need 1 midstream urine sample and this high CFU to be diagnosed with asymptomatic bacteriuria. Now, remember what we've talked about with catheters, how that puts you at an increased risk for urinary tract infection no matter what your age? So, if you have a catheter and you don't show signs of a urinary tract infection, the bar is much lower.

    06:00 See there, 10 to the 2nd power of CFUs and a catheter? Then we're going to diagnose you with asymptomatic bacteriuria. Before we leave this section on colony-forming units, I want to give you 2 special notes. One for lower CFUs and one for higher CFUs. So let's start with lower CFUs. Patients with catheter-associated UTIs have lower CFUs. Higher CFU counts happen in the presence of contaminates. So if you see a really high CFU, make sure that sample was collected appropriately because an incorrectly collected specimen could cause contamination and an inappropriately high report of CFUs.


    About the Lecture

    The lecture Colony-forming Units (CFU) in Urine Culture (Nursing) by Rhonda Lawes is from the course Interpretation of Renal Lab Values (Nursing).


    Author of lecture Colony-forming Units (CFU) in Urine Culture (Nursing)

     Rhonda Lawes

    Rhonda Lawes


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