How Much Blood is Too Much?

Test tube for blood. How much blood is too much?I received a phone call today from someone whose job includes drawing blood from children at a top university medical center. He was concerned that too much was being taken from children to be healthy for them. Often blood from children is drawn in the same big tubes used for adults. He had looked for guidelines for maximum recommended amounts from children, but called when he was unable to find them.

I applaud parents, nurses, or phlebotomists acting as advocates for children when large-volume draws are ordered, to be sure that the amount of blood requested is carefully considered. In general it is preferable to withdraw the smallest amount that will still allow the best medical care for the child. One way to do this is to ask that the blood be drawn in pediatric (not adult) tubes, where possible using something we call micro-sampling techniques – equipment designed to use the least amount necessary for each test.

To parents I suggest asking to be sure it is truly necessary if someone suggests taking more than 1 ml of blood per pound of your child’s weight (or a maximum of 30 ml for larger children).

To the phlebotomist who called I gave a general rule of thumb I use as an outside limit for draws, because at these levels of blood loss there can be measurable health consequences. Draws in infants and children should not exceed 5% of their total blood volume in any 24 hour period, unless medically necessary. This is a generous upper limit. Ideally it should be less than 3% of the total blood volume, and where possible, micro-sampling techniques to reduce the amount further are preferred. Draws in infants and children should not exceed 10% of the total blood volume in any one-month period, unless medically necessary.

The total blood volume can be estimated from the age and weight of the child. As a rough estimate, for pre-term babies the amount is about 90 ml per kilogram of body weight. For term newborns it is about 80 ml per kg. For older babies, 1 to 12 months of age, estimate 75 ml per kg. For toddlers up to age 3 estimate 70 ml per kg. And for older children and teens, 65 ml per kg is a reasonable estimate.

These are general guidelines, and the individual medical situation should be taken into account. But think twice whenever adult techniques or equipment are suggested for children.

Dr. Alan Greene

Dr. Greene is a practicing physician, author, national and international TEDx speaker, and global health advocate. He is a graduate of Princeton University and University of California San Francisco.

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