Blood Cultures in Pneumonia

A 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 CMS (Centers for Medicaid and Medicare Services),¹ IDSA and American Thoracic Society,² JCAHO,³ and Surviving Sepsis Guidelines⁴ have recommended 2 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:

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 1 major or 2 minor criteria:

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.

Conclusions

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.

Author information

Andrew Grock, MD

Lead Editor/Co-Founder of ALiEM Approved Instructional Resources (AIR)
Faculty Physician, Greater Los Angeles VA Healthcare System
Assistant Professor of Emergency Medicine
UCLA Emergency Medicine Department

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