How Much Blood is too Much Guideline

When taking blood from a child for testing or research purposes, how much is recommended and what can be safely taken? Dr. Greene weighs in.

Often blood from children is drawn in the same big tubes used for adults. I applaud parents, nurses, or phlebotomists acting as advocates for children when large-volume draws are ordered, to be sure that the amount 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 smallest 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). Thirty ml is equivalent to 1 ounce.


I gave a general rule of thumb I use as an outside limit for blood draws, because at these levels of blood loss there can be measurable health consequences. Blood 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. Blood 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 of blood 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.

To put these numbers in perspective, I have attached the guidelines from several different hospitals and labs. As you will see, there is not universal agreement on the precise numbers, but there is a general awareness that it is best to minimize blood loss, and the numbers used at different centers fall within a narrow range.

Alliance Laboratory Services uses the following table:

Maximum Amounts of Blood to be Drawn From Patients Younger than 14 Years

Patient’s Weight PoundsPatient’s Weight Kilograms (approx.)Maximum Amount to be drawn at any one time (mL)Maximum Amount –cumulative to be drawn during a given hospital stay (1 month or less-mL)


At Harvard’s Mass General they use the following guidelines for research purposes:

  • Blood volume taken from children must be less than 3 cc/kg body weight per 8 week period. In studies where the direct benefit far outweighs this volume restriction, a full protocol must be submitted for review of the full committee, and the following guidelines will apply:
    • If more than 3 cc/kg body weight per 8 week period is required and justified by the potential benefits, up to 9 cc venous blood/kg body weight/8 week period may be considered in older children (e.g., not neonates, toddlers, etc.), with the latter figure being the absolute upper limit.
    • Any child involved in a study involving removal of venous blood in the range of 3-9 cc/kg body weight per 8 week period should be placed on iron supplementation therapy. It is recommended that a dose of 30 mg ferrous sulfate/kg/day in 3 divided doses be given. Such therapy should continue for at least 8 weeks and should be monitored by hemoglobin measurements.

The Guidelines of the NIH Clinical Center for research purposes are:

For pediatric patients, no more than 3 ml/kg. may be taken for research purposes in a single draw, and no more than 7 ml./kg. may be drawn over any six-week period. Investigators should consider further limiting blood drawing in patients with anemia or low cardiac output.

In instances of clinical needs, phlebotomy in excess of the above limits may be permitted.

At UCLA they use:

  1. No more than 2.5% of total blood volume may be drawn solely for research purposes (no benefit to the subject) within a 24-hour period. This is generally 2 ml/kg.
  2. No more than 5% of total blood volume may be drawn solely for research purposes (no benefit to the subject) within a 30-day period. This is generally 4 ml/kg.
  3. If the investigator requests blood in excess of this amount, he/she must provide a detailed justification for this, and describe what safeguards are in place to protect the subject from undue risk.
Last medical review on: August 28, 2008
About the Author
Photo of Alan Greene MD
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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