12.2 Diagnostic & Laboratory Testing

Interpreting Pediatric Laboratory Data

Laboratory interpretation in children starts with age-appropriate reference ranges. Neonates are born with a high hemoglobin that falls to a physiologic nadir around 2-3 months of age. Alkaline phosphatase rises dramatically during pubertal growth spurts and must not be misread as liver or bone disease. Creatinine runs low in children because of small muscle mass. Confirm which pediatric reference range the laboratory used, and judge trends over time rather than single values.

Protein Status Markers — and Their Limits

Serum albumin has a long half-life (~20 days) and is a negative acute-phase reactant, so it falls with inflammation, infection, nephrotic syndrome, liver disease, and fluid overload independent of nutrition. Prealbumin (transthyretin), with a half-life of ~2 days, responds faster but is equally inflammation-sensitive (and rises with renal failure and corticosteroids). Per ASPEN and the Academy of Nutrition and Dietetics, neither is validated as a nutrition marker; if they are used at all, interpret them alongside C-reactive protein (CRP) — a low prealbumin with a high CRP reflects inflammation, not protein depletion. Expect exam questions built on this caveat.

Anemia Workup

Begin with the complete blood count (CBC) and red-cell indices:

• Microcytic, hypochromic anemia (low MCV): iron deficiency is most common. Differentiate from thalassemia trait, which typically shows a normal or elevated RBC count, mild anemia, and normal iron studies, whereas iron deficiency shows low ferritin and a low RBC count.
• Macrocytic anemia (high MCV): vitamin B12 or folate deficiency — consider this with an exclusively breastfed infant of a B12-deficient mother, malabsorption, or metformin use.
• Ferritin is the standard marker of iron stores, but it is an acute-phase reactant: a "normal" ferritin during inflammation does not exclude iron deficiency. Pair it with CRP, or use iron studies — iron deficiency shows low serum iron, high total iron-binding capacity (TIBC), and low transferrin saturation. The reticulocyte count gauges the marrow's response to treatment.

Electrolytes, Refeeding Risk, and Renal Function

Children at risk for refeeding syndrome (severe malnutrition, anorexia nervosa, prolonged fasting) need baseline and serial phosphorus, potassium, and magnesium; the hallmark of refeeding is hypophosphatemia as feeding resumes. Interpret BUN and creatinine in pediatric context: creatinine is low in children with small muscle mass, so a value in the adult "normal" range may signal renal impairment in a small child. BUN rises with dehydration and high protein intake and falls with overhydration and poor protein status.

Glucose and HbA1c

Pediatric diabetes diagnostic thresholds mirror adult criteria: fasting plasma glucose ≥126 mg/dL, a random glucose ≥200 mg/dL with classic symptoms, or HbA1c ≥6.5% — confirmed on repeat testing unless symptomatic hyperglycemia is unequivocal. HbA1c also monitors long-term glycemic control.

Lipid Screening

The NHLBI Expert Panel and the American Academy of Pediatrics (AAP) recommend universal lipid screening at ages 9-11 years and again at 17-21 years, with targeted screening from age 2 for children with a family history of early cardiovascular disease or risk factors. Suspect familial hypercholesterolemia when LDL cholesterol is ≥190 mg/dL, or ≥160 mg/dL with a family history of premature cardiovascular disease or other risk factors.

Liver Function and Fat-Soluble Vitamins

ALT, AST, bilirubin, and GGT assess hepatic function; remember that alkaline phosphatase elevations may simply reflect growth. Vitamin D status is measured as 25-hydroxyvitamin D: common pediatric cutoffs define deficiency as <20 ng/mL and insufficiency as 20-29 ng/mL. Children with cystic fibrosis, cholestatic liver disease, or other fat-malabsorption states need routine monitoring of fat-soluble vitamins A, D, E, and K (vitamin E often reported as a ratio to total lipids), because malabsorbed fat carries these vitamins with it.

Trace Elements

Zinc has no reliable single test — serum zinc falls with inflammation, fasting, and hypoalbuminemia; alkaline phosphatase, a zinc-dependent enzyme, serves as an indirect functional clue. Selenium and copper are monitored in patients on long-term parenteral nutrition (PN), where both deficiency and toxicity occur.

Inflammation Confounds Everything

CRP and erythrocyte sedimentation rate (ESR) mark inflammation, which lowers albumin, prealbumin, zinc, and vitamin A while raising ferritin and copper. Never interpret these protein or micronutrient values in isolation during acute illness.

Disease-Specific Tests

• Sweat chloride: the diagnostic test for cystic fibrosis — ≥60 mmol/L is positive, 30-59 mmol/L is borderline, and <30 mmol/L makes CF unlikely.
• Fecal elastase-1: <200 µg/g indicates exocrine pancreatic insufficiency (cystic fibrosis, chronic pancreatitis); it is not affected by pancreatic enzyme supplements taken during testing.
• Celiac serology: tissue transglutaminase IgA (tTG-IgA) plus total IgA — total IgA is needed because IgA deficiency falsely lowers tTG-IgA; test while the child is still on a gluten-containing diet.
• Stool alpha-1-antitrypsin: elevated in protein-losing enteropathy. Fecal calprotectin: elevated in inflammatory bowel disease, helping separate IBD from functional abdominal pain.
• Metabolic labs: ammonia, lactate, plasma amino acids, and acylcarnitine profiles evaluate inborn errors of metabolism (IEM).
• Urinalysis: specific gravity reflects hydration; ketones appear with poor intake or diabetes; proteinuria suggests renal losses.

Pediatric Lab Reference Table

Practical Ordering Strategy

Order labs to answer a specific assessment question, establish baselines before intervention, and follow trends — a prealbumin climbing toward normal alongside a falling CRP is far more meaningful than any single value.