Cultural awareness; small volumes of blood for culture cause under-detection of invasive infections

Alasdair P.S Munro, F2 Doctor,
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Other Contributors:

April 03, 2014

We read with interest your article demonstrating surprisingly low levels of culture-confirmed invasive bacterial infections in children.[1] Whilst we agree better strategies are needed for separating low risk, febrile children from those with invasive-infections, we believe there is another significant factor contributing to their apparent low rates.

Large studies demonstrate that blood-culture volumes are frequently inadequate in children.[2] It is well documented that the most significant factor in yielding an organism from culture is the volume of blood sampled.[3, 4] Insufficient volumes lead to large numbers of false- negatives, and it is estimated that 50% of cultures taken from children are of inadequate volume.[2]

We audited blood-culture volumes locally, analysing 66 patient samples (target volumes from previous studies [2]). We documented an overall compliance of 33%. Significantly this included 0% compliance in the >10yr age-group (N=11). Similar volumes were taken from 14 year- olds as 1 month-olds (Mean 1.92ml vs 1.00ml respectively), despite the >10-fold difference in circulating volume and haemodilutionary effect on bacterial concentrations. From our intervention we learned this was largely due to a lack of education of how much blood was necessary from different sized children. Better education is clearly needed.

Your article mentions standard practice taking "at least 1 - 2ml blood"[1], however for at least the >6000 tests performed in the 5-15 year-olds, 1-2ml of blood would be insufficient to produce a reliable result. Recent recommendations even suggest for children >36kg, an adult regime of cultures should be performed (40-60ml).[5]

Even taking into account "modern technologies [making] blood volumes less problematic"[1] our audit shows many clinicians remain unaware how much blood it is necessary to take. Considering that children remain difficult to venesect due to their small veins and poor compliance, the positive yield of blood-cultures is certain to underestimate the true number of invasive-infections in children.

References

1. Le Doare K, Nichols A-L, Payne H, et al. Very low rates of culture -confirmed invasive bacterial infections in a prospective 3-year population-based surveillance in Southwest London. Arch Dis Child Published Online First: February 19th 2014. doi:10.1136/archdischild-2013- 305565

2. Connell TG, Rele M, Cowley D, Buttery JP, Curtis N, How reliable is a negative blood culture result? Volume of blood submitted for culture in routine practice in a children's hospital, Pediatrics. 2007;119(5):891.

3. Mermel LA, Maki DG, Detection of bacteremia in adults: consequences of culturing an inadequate volume of blood, Ann Intern Med. 1993;119(4):270.

4. Ilstrup DM, Washington JA 2nd.The importance of volume of blood cultured in the detection of bacteremia and fungemia. Diagn Microbiol Infect Dis. 1983;1(2):107

5. Ellen Jo Baron et al, A guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2013 recommendations by the Infectious Diseases Society of America (IDSA) and the American Society for Microbiology, Clinical Infectious Diseases, 2013 Aug; 57 (4) : e22- e121.

Conflict of Interest:

None declared

Conflict of Interest

None declared