Volume of blood samples for culture
Blood sample volume is the most significant factor affecting the detection of organisms in bloodstream infection. There is a direct relationship between blood volume and yield, with approximately a 3% increase in yield per millilitre of blood cultured. False negative blood culture results may occur if inadequate blood sample volumes are obtainedConnell, 2007.
Studies suggest that the number of organisms present in bacteraemia in adults is often less than 1 x 103 colony forming units per litre (cfu/L)Mermel, 1993. Most modern commercial blood culture systems suggest, for adults, approximately 8 to 10 mL of blood be added to each bottle (ie 16 to 20 mL per 2-bottle set)Mylotte, 2000.
Data regarding the optimum blood volume per set for neonates and children are less certain and depend on patient weight (and to some extent, age). The magnitude of bacteraemia is usually higher in neonates and children than in adults; consequently, the sensitivity of detection is less affected by smaller blood sample volumes. Follow local policies and procedures where applicable, including the amount of blood per bottle recommended by the commercial blood culture system. Paediatric bottles are available, which are optimised for small volumes. A general guide for total blood sample volumes (distributed between each bottle if collecting in multiple bottles) is approximatelyO'Hagan, 2021:
- children 1 year to younger than 18 years: 1 mL per year of age
- children younger than 1 year who weigh 5 kg or more: 1 mL
- children younger than 1 year who weigh less than 5 kg: 0.5 to 1 mL
- children who weigh less than 1 kg: seek expert advice.