Normal AST and ALT Values for a Nine-Month-Old Infant
For a healthy nine-month-old infant, normal AST values are approximately 16–55 IU/L, and normal ALT values are approximately 1–23 IU/L for newborns transitioning to 10–28 IU/L by late infancy, with boys typically showing slightly higher values than girls. 1, 2
Age-Specific Reference Ranges
AST (Aspartate Aminotransferase)
- The combined reference interval for AST in newborns and infants is 16.1–55.4 IU/L, established through robust methodology in Ethiopian pediatric populations 2
- AST levels decrease with increasing age throughout infancy and early childhood 1
ALT (Alanine Aminotransferase)
- For infants (after the newborn period), the normal ALT range is approximately 10–28 IU/L 2
- Newborns have a slightly different range of 1.2–23.1 IU/L, which transitions to the infant range by several months of age 2
- Before 18 months of age, the 95th percentile cutoffs are 60 IU/L for boys and 55 IU/L for girls, though these represent upper screening thresholds rather than typical normal values 1
- After 18 months, these screening cutoffs decrease to 40 IU/L for boys and 35 IU/L for girls 1
Sex Differences in Normal Values
- Boys consistently show higher ALT levels than girls, even in infancy 1
- This sex difference becomes more pronounced after 18 months of age but is present from early infancy 1
- The difference reflects normal physiological variation rather than pathology 1
Clinical Context and Interpretation
What Constitutes "Elevated" in This Age Group
- For clinical screening purposes in children aged 9–11 years, ALT values ≥26 IU/L for boys and ≥22 IU/L for girls are considered elevated, though these thresholds are designed for NAFLD screening in older children 3
- In acute infectious diseases, ALT >60 IU/L is considered clinically significant elevation requiring follow-up 4, 5
- Isolated elevation of serum aminotransferases of at least 1.5 times above the norm for age is considered abnormal when it persists for at least 3 months 6
Important Caveats for Nine-Month-Olds
- Respiratory infections are the most common cause of transient ALT elevation in infants under 12 months, accounting for 45% of cases 4
- Half of all children with infectious disease-related ALT elevation are younger than 12 months, making this a common finding in this age group 4
- Isolated elevation of aminotransferases in healthy-appearing young children is mostly a benign condition that usually resolves within a year 6
- Normal values can vary based on the laboratory method used, so always compare results to the specific laboratory's reference range 2
When to Investigate Further
- If ALT or AST remains elevated for >3 months with values ≥1.5 times the upper limit of normal, further investigation is warranted 6
- If initial AST or ALT values exceed 200 IU/L or 125 IU/L respectively, more urgent evaluation is needed 5
- Ensure creatine phosphokinase (CK), gamma-glutamyltransferase (GGT), alkaline phosphatase, and bilirubin remain normal before attributing elevation solely to hepatic causes 6
Common Pitfalls to Avoid
- Do not use adult reference ranges for infants, as they are significantly different and will lead to misinterpretation 1, 2
- Do not ignore sex differences, even at nine months of age—boys naturally have higher values 1
- Do not immediately pursue extensive workup for mild elevations in the context of recent viral illness, as these typically resolve spontaneously within weeks to months 4, 6
- Do not assume all laboratories use the same reference ranges—always verify the specific laboratory's normative data 2