Before taking a blood serum sample it is important to asses the calf for dehydration. If a blood serum sample is taken on a dehydrated calf you will get a false reading, says Max Thornsberry, veterinarian and dairy technical specialist with Milk Specialties Global Animal Nutrition and Mid America Veterinary Consulting. Always do a skin tenting test prior to drawing a blood sample. Read these tips for evaluating calf dehydration.