Do endocrinologists use the Insulin-like Growth Factor 1 (IgF-1) lab result number, the z score, or both to interpret labs?

Interpretation of IGF-1 Lab Results in Endocrinology Practice

Endocrinologists use both the absolute IGF-1 lab result number and the age/sex-specific z-score (or SDS - standard deviation score) to interpret IGF-1 labs, with the z-score being particularly important for accurate clinical interpretation.

Primary Considerations for IGF-1 Interpretation

• Z-scores/SDS values: Essential for accurate interpretation as IGF-1 levels vary significantly with age and sex
• Absolute values: Used alongside z-scores, particularly when evaluating for specific cutoff thresholds

Clinical Importance of Z-Scores

Z-scores provide critical context for IGF-1 interpretation because:

• IGF-1 levels naturally decline with age
• Sex-specific variations exist in normal ranges
• Z-scores standardize results across different populations and laboratories 1

Disease-Specific Interpretation Approaches

For Acromegaly

• Primary diagnostic approach: IGF-1 elevation (typically with z-score >+2.0) is sufficient for diagnosis in most clinically suspected cases 2
• Confirmatory testing: OGTT (oral glucose tolerance test) may be used when IGF-1 elevation is modest (less than twice upper limit of normal) with equivocal clinical features 2
• Treatment monitoring: Both absolute IGF-1 values and z-scores are used to track treatment efficacy 3

For Growth Hormone Deficiency

• Screening approach: IGF-1 z-scores are more valuable than absolute values, but neither is sufficient alone for diagnosis 4
• Diagnostic threshold: Very low IGF-1 levels (z-score <-3 in adults over age 28) in the context of documented pituitary disease suggest high probability of GH deficiency 4
• Multiple deficiencies: In patients with two or more additional pituitary hormone deficiencies, severely low IGF-1 has 99% probability of indicating GH deficiency 4

Pitfalls in IGF-1 Interpretation

1. Non-linearity with GH levels: The relationship between GH and IGF-1 is not linear, particularly at GH levels above 1 ng/ml 3
2. Confounding factors: IGF-1 levels are affected by:
   • Nutritional status (decreased in malnutrition)
   • Thyroid function (decreased in hypothyroidism)
   • Liver disease (altered production)
   • Energy availability (decreased in low energy states) 1
3. Assay variability: Different laboratories use different assays with varying reference ranges, making standardization challenging

Special Populations

Athletes and Female Athlete Triad

• Low IGF-1 is a marker of low energy availability in female athletes 1
• Monitoring both absolute values and z-scores helps assess metabolic status and response to nutritional interventions

Post-Traumatic Brain Injury

• IGF-1 levels alone (even as z-scores) have poor utility as screening tools for GH deficiency following TBI 5
• Dynamic GH stimulation testing remains necessary despite normal IGF-1 levels

Best Practice Approach

1. Always interpret IGF-1 with age/sex-specific z-scores
2. Consider clinical context and pre-test probability of disease
3. Use confirmatory testing (GH stimulation tests or OGTT) when indicated
4. Monitor trends over time rather than isolated values when assessing treatment response
5. Account for confounding factors that may alter IGF-1 independent of GH status

This approach ensures the most accurate clinical interpretation of IGF-1 results and appropriate patient management decisions.