Are These Laboratory Values Normal for a 15-Year-Old Female?
Yes, both an alkaline phosphatase (ALP) of 170 U/L and an absolute lymphocyte count of 3.6 × 10⁹/L fall within normal reference ranges for a healthy 15-year-old female and require no further workup in the absence of clinical symptoms. 1, 2
Alkaline Phosphatase: 170 U/L
Normal Adolescent Physiology
ALP levels are physiologically elevated throughout childhood and adolescence due to active skeletal growth, with values peaking during the pubertal growth spurt. 3, 2
In adolescent females, the normal reference range for ALP extends up to approximately 400 U/L during peak growth periods, making a value of 170 U/L well within expected limits. 1, 4
Bone-specific ALP represents 80-90% of total circulating ALP in children and adolescents, compared to only 50% in adults, explaining the higher baseline values in this age group. 3, 5
ALP levels peak earlier in females (around age 11-14) compared to males (around age 13-16), then gradually decline toward adult values by the late teens. 2, 4
When ALP Would Be Concerning
An ALP of 170 U/L would only warrant investigation if accompanied by hypophosphatemia (low serum phosphate), which would suggest rickets or X-linked hypophosphatemia (XLH). 3, 5
Pathologically elevated ALP (significantly above age-specific reference ranges) combined with elevated gamma-glutamyl transferase (GGT) would indicate cholestatic liver disease requiring immediate evaluation. 3, 5
In the absence of symptoms (bone pain, skeletal deformities, jaundice, hepatomegaly) or other abnormal laboratory findings, an isolated ALP of 170 U/L in a 15-year-old female represents normal bone turnover. 3, 2
Absolute Lymphocyte Count: 3.6 × 10⁹/L
Normal Reference Range
The normal reference range for absolute lymphocyte count in adolescents and adults is 0.7-4.6 × 10⁹/L, making a value of 3.6 × 10⁹/L completely normal. 1
During pregnancy (for comparison), the normal range narrows slightly to 1.0-3.6 × 10⁹/L in the third trimester, but this patient is not pregnant, so the broader reference range applies. 1
Lymphocyte counts show minimal age-related variation after early childhood, with the adult reference range applicable to adolescents aged 15 years. 1, 6
Clinical Context
An absolute lymphocyte count of 3.6 × 10⁹/L sits at the upper end of normal but does not constitute lymphocytosis (which would be >4.6 × 10⁹/L). 1
Mild elevations in lymphocyte counts can occur transiently after recent viral infections or inflammatory processes, but a value of 3.6 × 10⁹/L does not require investigation in an asymptomatic patient. 7
No further workup is indicated for this lymphocyte count unless the patient has clinical signs of infection, lymphadenopathy, hepatosplenomegaly, or constitutional symptoms. 1, 6
Key Clinical Pitfalls to Avoid
Do not use adult reference ranges when interpreting ALP in adolescents—age-specific pediatric ranges are essential to avoid unnecessary workup. 3, 2, 6
Do not assume all "high" or "low" laboratory flags represent clinically significant abnormalities; approximately 5% of healthy individuals will have values outside the central 95% reference range by statistical definition. 6
Do not order extensive hepatobiliary or hematologic workups for isolated laboratory values that fall within age-appropriate reference ranges in asymptomatic patients. 3, 6
If there were concern for pathology, the next step for elevated ALP would be to measure GGT to differentiate hepatic from bone sources, but this is not indicated when ALP is within the normal adolescent range. 3, 5