Chapter 30: PEDIATRIC LAB VALUES

NOTES: This section on pediatric lab values has some GREAT TIPS AND TRICKS to getting some easy points from the American Board of Pediatrics’ board exams. This is not 100% accurate. It has been simplified, and many of the values have been rounded up or down in order to make the memorization easier. For example, instead of giving a range of 96–111 for newborns’ serum chloride, and then 102–112 for older children, an overall range of 100–110 has been given.

COMPLETE BLOOD COUNT (CBC)

• LEUKOCYTES (AKA WBC): For newborns, the WBC can be as high as 30,000 on DOL 1.
• LYMPHOCYTES: The WBC differential in young children normally shows a predominance of lymphocytes over neutrophils. The opposite is true in adolescents and adults. The lower limit of normal for the absolute lymphocyte count in pre-adolescent children varies by age, but the minimal acceptable ALC should be at least 2000/mm³.
• PEARL: Be sure to distinguish between the percentage and absolute counts of neutrophils and lymphocytes. The absolute count is the (percent/100)*(total WBC). So, for example, if the WBC = 14000/mm³ and there are 60% lymphocytes and 35% neutrophils, the absolute neutrophil count (ANC) = (35/100)*14000 = 4900/mm³, and the absolute lymphocyte count (ALC) = (60/100)*14000, or 8400/mm³.
• HEMOGLOBIN
  • NEWBORN: Anemia in a newborn is indicated by any hemoglobin value LESS THAN 13.
  • NADIR: Occurs at approximately 6 weeks of age. The value should be about 9.
• PLATELETS
  • NEWBORN: Consider thrombocytopenia in a newborn to be any platelet count LESS THAN 150. Look for clues in the history that point towards maternal ITP as etiology (this can last weeks).
  • OLDER KIDS: For children 2 months and older, set the lower limit of normal at approximately 200–250 in your mind.

COAGULATION STUDIES

• PARTIAL THROMBOPLASTIN TIME (PTT): Consider any value > 45 in a coagulation study to be prolonged.
• PROTHROMBIN TIME (PT): Consider any value > 15 to be prolonged.
  • NEWBORNS: May have a prolonged PT up to 9 months of age.

NORMAL PEDIATRIC ELECTROLYTE VALUES

• SODIUM: 135 – 145
• URINE SODIUM: A value of < 20 should indicate a low urinary sodium. The kidneys are either holding onto sodium tightly, or they are not holding onto water tightly enough. If you get confused, it’s probably in your best interest to assume that a low urine sodium reflects low/decreased sodium excretion (i.e., a low FeNA).
• POTASSIUM: 3.5 – 5
• CHLORIDE: 100 – 110
• URINE CHLORIDE: Use 15 as your cut-off. > 15 is high, < 15 is low.
• BICARBONATE LEVEL:
  • NEWBORNS: May have a normal bicarbonate level as low as 17. Sources vary on this, but use 22 as the upper limit of normal for a full-term newborn. Use 20 as the upper limit of normal for a premature newborn.
  • CHILDREN: 20–24. For ABGs, use 24 as the standard.
• BLOOD UREA NITROGEN (BUN): Use 18 as the upper limit of normal
• CREATININE: From birth to ~ DOL 10, the level reflects mom’s creatinine, so up to 1.3 is probably normal. After that, use the following as an approximate guide to the upper limit of normal:
  • 1 month = ~0.1
  • 1 year = ~ 0.3
  • 5 years = ~ 0.5
  • 9 years = ~ 0.6
  • 13 years = ~ 0.8
• CALCIUM
  • TOTAL CALCIUM: 8 – 11 mg/dl
  • IONIZED CALCIUM: Upper limit = 5 mg/dl
• MAGNESIUM: 1.5 – 2.5 mg/dl
• PHOSPHORUS: 3.5 – 6 mg/dl (adults 3–5)
• ALBUMIN: 3.5 – 5 g/dl
• TOTAL PROTEIN: ~ 5 – 8 g/dl
• AST: < 40
  • LOW-YIELD FACTS: Newborns may have a normal AST value of up to 120. Children aged 10 days to 2 years can have a normal value up to 80. After 2 years of age, though, it should definitely be < 40.
• ALT: < 45