A study by Sara Keim published in the April 6th issues of Pediatrics found that 10 out of 102 breast milk samples that were purchased online contained 10% or more cow's milk, potentially putting lactose intolerant infants at risk. The samples analyzed came from several sites, including the Breast, Eats on Feets, and Human Milk 4 Human Babies. In a previous study using the same samples (all purchased in 2012), Sara and her teem at Nationwide Children's Hospital Medical Center in Columbus found 75% of samples were contaminated with virus or bacteria. (This isn't necessarily as alarming as it sounds; people in general harbor bacteria and viruses so any time you have a human fluid packaged and shipped by humans, it's bound to contain bacteria from time to time.)
The researchers used a molecular test to determine the amount of cow's milk, which suggested a notable number of the sellers intentionally added cows milk or infant formula to the breast milk. An online donor is a complete unknown, and it is hard to determine if the sellers would be entirely honest about their product. This illustrates the need for parents to be cautious about their supplier, especially if your baby has cow milk allergy.