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Bryan Coburn, MD, PhD, discusses the clinical examination for bacteremia.
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Bryan Coburn, MD, PhD, discusses the clinical examination for bacteremia.
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Segment:0 .
[ Music ] >> Hello and welcome to JAMAevidence, our monthly podcast focused on core issues in evidence-based medicine. I'm David Simel, the Editor of the Rational Clinical Examination series and Professor of Medicine at the Durham Veterans' Affairs Medical Center in Duke University. Today, we're discussing the clinical evaluation for bacteremia. Joining me to talk about this topic is Dr. Bryan Coburn, Assistant Professor of Medicine at the University of Toronto.
Dr. Coburn, as we're recording this, we're one month into welcoming our new house staff to the medical center, and many of them have been introduced to the fever workup. Tell us if you have a mental checklist of risk factors for bacteremia that you would suggest they review when they get a call that their patient has a fever. >> I think the most important thing to consider when approaching a patient with fever and considering the diagnosis of bacteremia is what the primary diagnosis of infection would be.
In general, and this is particularly true in hospitalized in-patients, bacteremia is secondary to a primary focus of infection such as a central line infection, pneumonia, skin and soft-tissue infection, or some other cause. And my first thought is always to go through the mental checklist of what the possible focal infections would be in a febrile hospitalized in-patient. So, primary bacteremias like infective endocarditis are pretty rare in hospitalized in-patients, and I go through a relatively short list of primary foci of infection when approaching a febrile in-patient.
Medical patients, the most common causes of fever associated with bacteremia would be pneumonia and catheter-associated bloodstream infections. Urinary tract infections are also common causes of fever in in-patients. Soft-tissue infections like pressure ulcers are common in in-patients, but relatively rare causes of bacteremia. So, the first thing I do is stratify the risk of bacteremia based on what I feel the likely primary diagnosis is. And that informs my decision to do a blood culture or not.
>> Well, in the adult hospitalized patient, just how likely is it that bacteremia is going to be the cause of their fever? >> The rate of true positive blood cultures in febrile in-patients is about 15%. True positives being those positive for an organism that is a pathogen responsible for infection as opposed to a contaminant. This is hospitalized in-patients. True positives in this population represent a heterogeneous group of sources of infection such as those that I listed before. Outpatients have rates of infection and bacteremia that depend on their reason for presenting, but hospitalized in-patients is about 15%.
>> Well, when the nurse calls me about temperature elevation in a patient, why can't I just use the degree of temperature elevation alone as a sign of bacteremia? >> Unfortunately, temperature elevation isn't a particularly reliable indicator of bacteremia and there's lots of interesting biology behind this. There are both patient-specific factors and pathogen-specific factors that can affect the degree of temperature elevation in response to infection in bacteremia. Patients may be on medications that affect your blood pressure, Tylenol is a good example.
Different pathogens have different degrees of pathogenicity and virulence determinants that can promote fever so this can depend on the pathogen as well. And as a result of this, and likely other factors, there's no reliable relationship between the degree of fever and the likelihood of bacteremia. >> Well, it seems counterintuitive that the amount of temperature elevation alone is not useful. So, there has to be some findings that tell us who is most likely to have bacteremia. Are there any? >> Yeah, and I think that this is something that becomes evident with years of practice, but one of them is simply the clinical impression.
If you have a high clinical suspicion of bacteremia, particularly experienced physicians, that is actually a relatively good indicator of the likelihood of bacteremia. Another strong indicator of bacteremia which I use commonly is shaking chills or rigors. I'm immediately suspicious of a patient who has shaking chills or rigors and a blood culture will be part of my workup to assess the possibility of bacteremia in that patient population as well. >> Well, I'm glad to hear that clinical impression is important and that way I can tell my house staff they have something to look forward to.
So, let's look at this from the opposite perspective. Are there any findings or combinations of findings that tell us that the probability of bacteremia is so low that we don't need to request blood cultures? >> Depending on the patient population, all comers with acute infection, including those being evaluated in the emergency department, fever alone is not a particularly good indicator, as we discussed, and the absence of other signs or symptoms or laboratory findings suggestive of infection make bacteremia unlikely.
The systemic inflammatory response syndrome criteria, which were until relatively recently a common criterion used to establish the risk of sepsis or the diagnosis of sepsis, the absence of fulfilling this criteria so fewer than two of the criteria of temperature elevation, elevated respiratory rate, or low CO2, elevated or decreased white blood cell count, and tachycardia, fewer than two of those criteria are met, bacteremia was found to be unlikely in one study. Several other studies have used multivariable predictors that have some overlap with the SIRS criteria that have been useful for decreasing the probability of bacteremia.
I think the key message, of course, is that no test is perfect for ruling out bacteremia and the first question is what your pre-test suspicion about bacteremia is and that is largely determined by the clinical syndrome that the patient is presenting with. So, there's no perfect test, but both SIRS and some multivariable scores are useful for decreasing the likelihood of bacteremia significantly. >> So, you can go through a checklist of temperature elevation, elevated respiratory rate or low bicarb, and elevated or decreased white blood cell count and tachycardia, and if they have only one or two of those it makes bacterial infection unlikely.
>> It makes bacteremia unlikely, yeah. >> Okay. So, are there some settings where you would recommend always getting a blood culture? >> Yes, and I think that it's important to add a caveat to the discussion around blood cultures. There are some conditions when being considered that require a blood culture. Infective endocarditis is the key condition that is an example of this. The diagnosis of infective endocarditis, in many cases, requires a positive blood culture so if this diagnosis is suspected for one of a large number of reasons, blood cultures should be ordered.
A bacterial meningitis is another important example. The rates of positive blood cultures for bacteremia are very high in this population, over 60% in some studies. And, more importantly, the culture result that may be obtained by drawing a blood culture may give you an microbiologic etiology for bacterial meningitis that you may not obtain by doing a lumbar puncture, especially if the patient has had antibiotics. So, it may be an important and, in fact, the only way to determine an etiologic organism in that setting.
I always order blood cultures when I suspect a catheter-related bloodstream infection in in-patients. Often, this is part of a workup for a fever without a clear alternate focus in a hospitalized in-patient or ICU patient. That would be another indication. >> Well, it sounds like there are some patients where you would almost always get a blood culture when they have a fever, but if I'm a new intern it seems an obvious approach to me would be to just request a blood culture on every hospitalized patient the first time they have a fever and that way we could always say that I tried to rule out bacteremia.
Is that okay? >> I think this is an important point for discussion. There is a bit of a reflex. It's almost a pre-built response when you get a call about a fever as a medical house staff or a surgical house staff to request a chest x-ray, blood cultures, and urine cultures. If blood cultures were a perfect test and had no false positives, this strategy might be defensible, but a large number of positive blood cultures, as many as half in some surveys of hospitalized in-patients, are positive for contaminates such as coagulase-negative staphylococci.
And the culturing of that organism, which is usually introduced into the culture bottle at the time of drawing of the blood culture from the skin, is associated with unnecessary testing, prolongation of hospital stay, increased cost, toxicity associated with drugs administered for a condition that the patient doesn't have. So, false-positive blood cultures have real potential harms and need to be considered when your pre-test suspicion of bacteremia is actually very low. The likelihood of getting a false positive may, in some cases, be as high or higher than the likelihood of having a true positive culture.
>> So, Bryan, while our discussion has centered around blood cultures, we don't want to be out of date. Are there any new tests coming on that look promising for identifying the patient who has bacteremia? >> Absolutely. You know, it's actually quite an interesting time, the last 10 years of progress in microbiology and medical microbiology. Rapid diagnostic tests, which are often increasingly sophisticated molecular tests, can provide a result before an organism is even cultured in some cases and this result has two features.
One is that it's very rapid and two it often provides the species identification that allows us to distinguish between a false positive or a likely false-positive blood culture and a true positive blood culture. The advent of these technologies may allow us to make a decision about false versus true positive cultures before initiating empiric therapy. As these tests get ruled out and become more widely available, our algorithm and approach to ordering blood cultures may change because the relative rate of true positives may increase relative to false positives.
And as we learn how these tests work after implementation, it may change how this approach to blood culture ordering should work. >> Well, Bryan, this is a really complicated topic and I'm glad that my clinical experience or my clinical impression may be finally paying off. I want to thank you for this interesting discussion. Is there anything else you would like our listeners to know when trying to decide if their patient has bacteremia? >> I think that for me it's actually what to do when you obtain a result of a blood culture.
If you're considering doing a blood culture, you have to be prepared to act on both a positive and negative result. And the most important way to act on that result is to consider what it means in the context of the patient that you're evaluating. A microbiologic result, whether it's positive or negative, is an important opportunity to reassess your differential diagnosis. In the setting of a positive culture, consider whether the pathogen or organism that is isolated is consistent with the infectious syndrome you suspect or have otherwise diagnosed.
If it's negative, ask yourself whether this really changes the probability of your suspected diagnosis and whether this should affect your management. This is going to help you decide whether the test will be useful. If you've imagined what the scenario will be when you get the result, it's going to inform your decision to order the test. >> Thank you, Dr. Coburn, for talking with us today about bacteremia. More information about this topic is available in the Rational Clinical Exam and on our website, JAMAevidence.com, where you can listen to our entire roster of podcasts.
I'm David Simel and I'll be back with you soon for another edition of JAMAevidence.
Segment:0 .
[ Music ] >> Hello and welcome to JAMAevidence, our monthly podcast focused on core issues in evidence-based medicine. I'm David Simel, the Editor of the Rational Clinical Examination series and Professor of Medicine at the Durham Veterans' Affairs Medical Center in Duke University. Today, we're discussing the clinical evaluation for bacteremia. Joining me to talk about this topic is Dr. Bryan Coburn, Assistant Professor of Medicine at the University of Toronto.
Dr. Coburn, as we're recording this, we're one month into welcoming our new house staff to the medical center, and many of them have been introduced to the fever workup. Tell us if you have a mental checklist of risk factors for bacteremia that you would suggest they review when they get a call that their patient has a fever. >> I think the most important thing to consider when approaching a patient with fever and considering the diagnosis of bacteremia is what the primary diagnosis of infection would be.
In general, and this is particularly true in hospitalized in-patients, bacteremia is secondary to a primary focus of infection such as a central line infection, pneumonia, skin and soft-tissue infection, or some other cause. And my first thought is always to go through the mental checklist of what the possible focal infections would be in a febrile hospitalized in-patient. So, primary bacteremias like infective endocarditis are pretty rare in hospitalized in-patients, and I go through a relatively short list of primary foci of infection when approaching a febrile in-patient.
Medical patients, the most common causes of fever associated with bacteremia would be pneumonia and catheter-associated bloodstream infections. Urinary tract infections are also common causes of fever in in-patients. Soft-tissue infections like pressure ulcers are common in in-patients, but relatively rare causes of bacteremia. So, the first thing I do is stratify the risk of bacteremia based on what I feel the likely primary diagnosis is. And that informs my decision to do a blood culture or not.
>> Well, in the adult hospitalized patient, just how likely is it that bacteremia is going to be the cause of their fever? >> The rate of true positive blood cultures in febrile in-patients is about 15%. True positives being those positive for an organism that is a pathogen responsible for infection as opposed to a contaminant. This is hospitalized in-patients. True positives in this population represent a heterogeneous group of sources of infection such as those that I listed before. Outpatients have rates of infection and bacteremia that depend on their reason for presenting, but hospitalized in-patients is about 15%.
>> Well, when the nurse calls me about temperature elevation in a patient, why can't I just use the degree of temperature elevation alone as a sign of bacteremia? >> Unfortunately, temperature elevation isn't a particularly reliable indicator of bacteremia and there's lots of interesting biology behind this. There are both patient-specific factors and pathogen-specific factors that can affect the degree of temperature elevation in response to infection in bacteremia. Patients may be on medications that affect your blood pressure, Tylenol is a good example.
Different pathogens have different degrees of pathogenicity and virulence determinants that can promote fever so this can depend on the pathogen as well. And as a result of this, and likely other factors, there's no reliable relationship between the degree of fever and the likelihood of bacteremia. >> Well, it seems counterintuitive that the amount of temperature elevation alone is not useful. So, there has to be some findings that tell us who is most likely to have bacteremia. Are there any? >> Yeah, and I think that this is something that becomes evident with years of practice, but one of them is simply the clinical impression.
If you have a high clinical suspicion of bacteremia, particularly experienced physicians, that is actually a relatively good indicator of the likelihood of bacteremia. Another strong indicator of bacteremia which I use commonly is shaking chills or rigors. I'm immediately suspicious of a patient who has shaking chills or rigors and a blood culture will be part of my workup to assess the possibility of bacteremia in that patient population as well. >> Well, I'm glad to hear that clinical impression is important and that way I can tell my house staff they have something to look forward to.
So, let's look at this from the opposite perspective. Are there any findings or combinations of findings that tell us that the probability of bacteremia is so low that we don't need to request blood cultures? >> Depending on the patient population, all comers with acute infection, including those being evaluated in the emergency department, fever alone is not a particularly good indicator, as we discussed, and the absence of other signs or symptoms or laboratory findings suggestive of infection make bacteremia unlikely.
The systemic inflammatory response syndrome criteria, which were until relatively recently a common criterion used to establish the risk of sepsis or the diagnosis of sepsis, the absence of fulfilling this criteria so fewer than two of the criteria of temperature elevation, elevated respiratory rate, or low CO2, elevated or decreased white blood cell count, and tachycardia, fewer than two of those criteria are met, bacteremia was found to be unlikely in one study. Several other studies have used multivariable predictors that have some overlap with the SIRS criteria that have been useful for decreasing the probability of bacteremia.
I think the key message, of course, is that no test is perfect for ruling out bacteremia and the first question is what your pre-test suspicion about bacteremia is and that is largely determined by the clinical syndrome that the patient is presenting with. So, there's no perfect test, but both SIRS and some multivariable scores are useful for decreasing the likelihood of bacteremia significantly. >> So, you can go through a checklist of temperature elevation, elevated respiratory rate or low bicarb, and elevated or decreased white blood cell count and tachycardia, and if they have only one or two of those it makes bacterial infection unlikely.
>> It makes bacteremia unlikely, yeah. >> Okay. So, are there some settings where you would recommend always getting a blood culture? >> Yes, and I think that it's important to add a caveat to the discussion around blood cultures. There are some conditions when being considered that require a blood culture. Infective endocarditis is the key condition that is an example of this. The diagnosis of infective endocarditis, in many cases, requires a positive blood culture so if this diagnosis is suspected for one of a large number of reasons, blood cultures should be ordered.
A bacterial meningitis is another important example. The rates of positive blood cultures for bacteremia are very high in this population, over 60% in some studies. And, more importantly, the culture result that may be obtained by drawing a blood culture may give you an microbiologic etiology for bacterial meningitis that you may not obtain by doing a lumbar puncture, especially if the patient has had antibiotics. So, it may be an important and, in fact, the only way to determine an etiologic organism in that setting.
I always order blood cultures when I suspect a catheter-related bloodstream infection in in-patients. Often, this is part of a workup for a fever without a clear alternate focus in a hospitalized in-patient or ICU patient. That would be another indication. >> Well, it sounds like there are some patients where you would almost always get a blood culture when they have a fever, but if I'm a new intern it seems an obvious approach to me would be to just request a blood culture on every hospitalized patient the first time they have a fever and that way we could always say that I tried to rule out bacteremia.
Is that okay? >> I think this is an important point for discussion. There is a bit of a reflex. It's almost a pre-built response when you get a call about a fever as a medical house staff or a surgical house staff to request a chest x-ray, blood cultures, and urine cultures. If blood cultures were a perfect test and had no false positives, this strategy might be defensible, but a large number of positive blood cultures, as many as half in some surveys of hospitalized in-patients, are positive for contaminates such as coagulase-negative staphylococci.
And the culturing of that organism, which is usually introduced into the culture bottle at the time of drawing of the blood culture from the skin, is associated with unnecessary testing, prolongation of hospital stay, increased cost, toxicity associated with drugs administered for a condition that the patient doesn't have. So, false-positive blood cultures have real potential harms and need to be considered when your pre-test suspicion of bacteremia is actually very low. The likelihood of getting a false positive may, in some cases, be as high or higher than the likelihood of having a true positive culture.
>> So, Bryan, while our discussion has centered around blood cultures, we don't want to be out of date. Are there any new tests coming on that look promising for identifying the patient who has bacteremia? >> Absolutely. You know, it's actually quite an interesting time, the last 10 years of progress in microbiology and medical microbiology. Rapid diagnostic tests, which are often increasingly sophisticated molecular tests, can provide a result before an organism is even cultured in some cases and this result has two features.
One is that it's very rapid and two it often provides the species identification that allows us to distinguish between a false positive or a likely false-positive blood culture and a true positive blood culture. The advent of these technologies may allow us to make a decision about false versus true positive cultures before initiating empiric therapy. As these tests get ruled out and become more widely available, our algorithm and approach to ordering blood cultures may change because the relative rate of true positives may increase relative to false positives.
And as we learn how these tests work after implementation, it may change how this approach to blood culture ordering should work. >> Well, Bryan, this is a really complicated topic and I'm glad that my clinical experience or my clinical impression may be finally paying off. I want to thank you for this interesting discussion. Is there anything else you would like our listeners to know when trying to decide if their patient has bacteremia? >> I think that for me it's actually what to do when you obtain a result of a blood culture.
If you're considering doing a blood culture, you have to be prepared to act on both a positive and negative result. And the most important way to act on that result is to consider what it means in the context of the patient that you're evaluating. A microbiologic result, whether it's positive or negative, is an important opportunity to reassess your differential diagnosis. In the setting of a positive culture, consider whether the pathogen or organism that is isolated is consistent with the infectious syndrome you suspect or have otherwise diagnosed.
If it's negative, ask yourself whether this really changes the probability of your suspected diagnosis and whether this should affect your management. This is going to help you decide whether the test will be useful. If you've imagined what the scenario will be when you get the result, it's going to inform your decision to order the test. >> Thank you, Dr. Coburn, for talking with us today about bacteremia. More information about this topic is available in the Rational Clinical Exam and on our website, JAMAevidence.com, where you can listen to our entire roster of podcasts.
I'm David Simel and I'll be back with you soon for another edition of JAMAevidence.
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