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Blood Cultures in Pneumonia


PneumoniaRULA 75 year old woman is found to have pneumonia. With a CURB-65 of 3 and a PORT score of 95, she is correctly treated her for community acquired pneumonia with Ceftriaxone and Azithromycin, and admitted. Unfortunately, the admitting service points out that no blood cultures were drawn! What is the evidence for this?

Originally from Clinical Monster blog

Official Guidelines and Recommendations

I have been told that CMS1 (Centers for Medicaid and Medicare Services), IDSA and American Thoracic Society,2 JCAHO,3 and Surviving Sepsis Guidelines4 have recommended two sets of blood cultures prior to antibiotics for anyone admitted for pneumonia.

Update: 2/17/14: As of January 1, 2014, the CMS and JCAHO are retiring blood cultures for non-ICU patients as a core measure (PN-3b measure). Also the Surviving Sepsis Campaign recommendations are for pneumonia patients with severe sepsis only. Thanks to Drs. Seth Trueger, Scott Weingart, and @DocERTrauma for this correction. 

What is the evidence for this?

Two studies retrospectively reviewed total blood culture yield. One5 had 1.4% true positive blood cultures with only 0.18% affecting patient management. The other6 yielded 5% true positives with 1.6% (18 patients) affecting patient management. By combing these studies we find a 0.7% chance of affecting patient care. That’s a NNT of 143! Additionally, patients with false positive cultures had significantly longer length of stays leading to increased cost.

Benenson RS et al. J Emerg Med (2007)

This retrospective review7 examined 684 patient charts with ED blood cultures and a discharge diagnosis of pneumonia. They found 23 (3.4%) true positive and 54 (7.8%) false positives. Of the true positives, 3 had their antibiotic regimen narrowed without anyone needing broader coverage. That’s a 0.4% chance of a blood culture drawn affecting patient management, or an NNT of 250! Interestingly, 18 additional patients could have been narrowed based on culture results, but were not. The authors recommended “eliminating blood cultures for CAP patients and obtaining blood cultures for HCAP patients presenting to the ED”

Kennedy M et al. Ann Emerg Med (2005)

This retrospective review8 demonstrated that of 414 ED blood cultures drawn for pneumonia, 29 (7%) were true positives and 25 (6%) were false positives/contaminants. Of the 29 true positives, 11 (2.7%) had their coverage narrowed, 4 (1%) had their coverage broadened, and another 8 could have been narrowed based on culture results but were not. In this study, the rate of blood culture results affecting patient care was higher at 3.6%. Of note, 3 of the 4 that needed broader coverage were from a nursing home, had MRSA, but had not been initially treated with hospital acquired pneumonia regimens.

Coburn B et al. JAMA (2012)

This literature search/meta-analysis9 investigated the risk of true positive blood cultures in immune-competent patients without suspicion for endocarditis. They recommended NOT to do a blood culture in the Low Risk Group due to the low pre-test probability of a true positive culture! Below is a summary of their findings:

Risk Group Pretest Probability of Positive Blood Culture Recommend Blood Culture? Diagnoses Included
Low <14% No Pneumonia, cellulitis
Medium 19-25% Yes Pyelonephritis
High 38-68% Yes Severe sepsis, septic shock, bacterial meningitis

In terms of risk factors for a true positive culture, the following were NOT statistically significant:

  • Subjective fever
  • Tachycardia alone
  • Elevated WBC count
  • Documented fever

The following WERE statistically significant:

  • Shaking chills
  • Hypotension
  • Requiring vasopressors
  • Neutrophil/lymphocyte ratio > 10
  • Presence of SIRS

Interestingly, having zero SIRS criteria had a LR 0.09 for a true positive culture.

Shapiro NI et al. J Emerg Med (2008)10

This prospective analysis included 3,730 pneumonia patients with blood cultures drawn in the ED or up to 3 hours after admission. In the derivation population, they found a statistically significant increased risk of positive blood culture with certain characteristics. After some moderately complicated calculations, they created Major Criteria and Minor Criteria and recommended a blood culture only if the patient has one major or two minor criteria:

Minor criteria Major criteria
Temperature > 101F (38C) Suspected endocarditis
Age > 65 years Temperature > 103F (40C)
Chills Indwelling vascular catheter
Systolic BP < 90 mmHg
WBC > 18k
Bands > 5%
Platelets < 150,000
Creatinine > 2 mg/dL

This approach had a negative predictive value for a true positive blood culture of 99.4% (95% CI 99-100%) and 99.1% (95% CI 98-100%) in the derivation group and validation group, respectively. Seven total patients were missed by the decision rule. Of those, five would have had no change in management, one should have received a culture by the prediction rule but was missed in the ED, and one received a blood culture for a fever > 3 hours after admission.

So what do these organizations actually say?

IDSA and ATS guidelines2 for Community Acquired Pneumonia (CAP)

These organizations recommend blood cultures for patients with CAP only if they have at least one for the following:

  • Admission to the intensive care unit
  • Cavitary infiltrates
  • Leukopenia
  • Chronic severe liver disease
  • Asplenia
  • Plural effusion
  • A positive pneumococcal urinary antigen test
  • Active alcohol abuse

Bonus Clinical Pearls

  1. If cultures are performed, the literature emphasizes that volume matters.11 There is a 3% increased yield in positive blood cultures per mL of blood obtained. They recommend at least 7 mL per blood culture bottle.
  2. Additionally, the optimal time to culture a patient is 1-2 hours prior to fever occurring.12 Multiple studies have shown no significant difference in results if the patient is cultured at Tmax vs hours before vs hours after. According to this data, you don’t need to rush to do cultures when someone is febrile.
  3. Another study13 showed that contaminated blood culture rates increased linearly with increased patient load in the ED.


  1. Obtaining blood cultures for non-ICU CAP are NO LONGER core measures per CMS and JCAHO as of January 1, 2014, finally in alignment with studies showing their low utility value in low risk patients. [Updated 2/17/14] 
  2. Blood cultures are not universally required for admitted CAP patients per IDSA/ATS, but required per CMS/JCAHO for ICU patients (PN-3a measure). [Updated 2/18/14]
  3. Cultures rarely affect management, but are recommended by the literature when certain criteria are met including sepsis and endocarditis.
  4. If blood cultures are obtained, clean well, don’t hurry, don’t worry wait until patient is febrile, and fill up bottles with at least 7 mL each.
Overview of Specifications of Measures Displayed on Hospital. Published December 14, 2006.
Mandell L, Wunderink R, Anzueto A, et al. Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clin Infect Dis. 2007;44 Suppl 2:S27-72. [PubMed]
Pneumonia National Hospital Inpatient Quality Measures. The Joint Commission.
Recommendations: Initial Resuscitation and Infection Issues.
Howie N, Gerstenmaier J, Munro P. Do peripheral blood cultures taken in the emergency department influence clinical management? Emerg Med J. 2007;24(3):213-214. [PubMed]
Kelly A. Clinical impact of blood cultures taken in the emergency department. J Accid Emerg Med. 1998;15(4):254-256. [PubMed]
Benenson R, Kepner A, Pyle D, Cavanaugh S. Selective use of blood cultures in emergency department pneumonia patients. J Emerg Med. 2007;33(1):1-8. [PubMed]
Kennedy M, Bates D, Wright S, Ruiz R, Wolfe R, Shapiro N. Do emergency department blood cultures change practice in patients with pneumonia? Ann Emerg Med. 2005;46(5):393-400. [PubMed]
Jourdan C, Artru F, Convert J, et al. [A rare and severe complication of meningeal hemorrhage: spinal arachnoiditis with paraplegia]. Agressologie. 1990;31(6):413-414. [PubMed]
Shapiro N, Wolfe R, Wright S, Moore R, Bates D. Who needs a blood culture? A prospectively derived and validated prediction rule. J Emerg Med. 2008;35(3):255-264. [PubMed]
Mermel L, Maki D. Detection of bacteremia in adults: consequences of culturing an inadequate volume of blood. Ann Intern Med. 1993;119(4):270-272. [PubMed]
Riedel S, Bourbeau P, Swartz B, et al. Timing of specimen collection for blood cultures from febrile patients with bacteremia. J Clin Microbiol. 2008;46(4):1381-1385. [PubMed]
Halverson S, Malani P, Newton D, Habicht A, Vander H, Younger J. Impact of hourly emergency department patient volume on blood culture contamination and diagnostic yield. J Clin Microbiol. 2013;51(6):1721-1726. [PubMed]
Andrew Grock, MD

Andrew Grock, MD

Lead Editor/Co-Founder of ALiEM Approved Instructional Resources (AIR)
Assistant Professor of Emergency Medicine
UCLA Emergency Medicine Department
Andrew Grock, MD

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  • Todd

    TJC core measure PN-3b appears to have been retired 1/1/14:

  • er doctor

    please double check but i think that though cms 3b is retired, the requirement for blood culture in ICU patients remains, ie pna 3a

  • andrew grock

    To start, I must profusely thank Dr. Lin, Dr. Weingart, Dr. Trueger, @erdoctrauma, and our two commentators below for correcting an oversight in my post. When I did this literature search in early January, I somehow completely missed CMS/JCAHO retiring the core measure of blood cultures pre-antibiotics for pneumonia patients being admitted to the floor. (

    Also, per Dr. Weingart the blood culture recommendation from surviving sepsis guidelines only applies to pneumonia patients with severe sepsis. (

    My sincerest apologies for this error. At the very least, I hope this post serves to present the data analyzing the utility of blood cultures; and I hope your valuable comments help educate people, who like me, missed the practice-changing guideline retirement!

    Interestingly, my hospital has yet to change its policy requiring blood cultures for admitted patients with pneumonia. Hopefully, our conversation can contribute to changing these hospital-based policies as well.

  • njoshi8

    I think that it is very clear based upon discussion and analysis of the post and comments that blood cultures in the ED is an area that is changing and guidelines vary based upon where the pt is and what their presentation presents like – all this makes it VERY challenging for us, the ED providers to know what to do and how to proceed! Especially when hospital policy defers from national guidelines which defer from EBM… very frustrating! But I am glad that we have blogs and twitter format out there to at least get the information out there as best as possible.

  • Lwazi

    In my unit and certainly when I refer a patient, we are very clear about when and when not to do blood cultures and accepting disciplines who insist on it are compelled to provide evidence otherwise do it themselves in their own budget. we dont have the financial resources to do unneccessary investigations so everything we do is evidence based and not just to mitigate litigation

  • Josh Farkas

    Great post.

    Would note that the IDSA/ATS guidelines for “CAP” are not applicable to patients with immunocompromise (including transplantation, chemotherapy, long-term high-dose steroid, congenital or acquired immunodeficiency). Likewise, some of the studies on blood cultures in CAP have excluded patients with immunocompromise.

    This post indirectly emphasizes that if we’re going to get blood cultures its important to get two sets of cultures in order to differentiate contamination (1/2 sets) vs true-positive (2/2 sets). If only one set is obtained it’s harder to make this differentiation up front, which may lead to inappropriate antibiotic use until speciation is returned some days later. So probably best to get either zero or two sets, depending on the situation.

    • andrew grock

      I agree Dr Farkas. These guidelines and some of the studies were limited to immunocompetent patients. Given that they are at higher risk for severe infections and less common infections, I personally would culture them. I do not know the utility of blood cultures per evidence based medicine though. According to ACEP guidelines on blood cultures in pneumonia – level B recommendation to not do blood cultures in CAP. They give a level C recommendation for blood cultures in CAP for “high risk” populations such as the ones you listed above.
      Also, I was really surprised at how many of the blood cultures drawn in these studies were contaminants. Typically half of the positive blood cultures were false positives, which one paper described as leading to more expensive/longer hospital stays. Along with helping identify false positives, two cultures also increases your chances of obtaining a true positive blood culture. Lastly, volume drawn matters. On review of the newest IDSA guidelines, they recommend 20-30 ml per 4 blood culture containers.

      • andrew grock

        Quick typo fix. In the third sentence I meant the high risk patients you listed with “they” not immunocompetent cap patients.