Laboratory Testing for Anorexia Nervosa
The American Psychiatric Association recommends obtaining a complete blood count and comprehensive metabolic panel (including electrolytes, liver enzymes, and renal function tests) as the core laboratory tests for evaluating patients with suspected anorexia nervosa. 1, 2
Essential Initial Laboratory Tests
Core Screening Panel
- Complete blood count (CBC) to detect anemia, leukopenia, thrombocytopenia, and other hematologic abnormalities that commonly occur in malnutrition 1, 2, 3
- Comprehensive metabolic panel including:
- Electrolytes (sodium, potassium, chloride, bicarbonate) to identify hyponatremia, hypokalemia, hypochloremia, and metabolic alkalosis, particularly in patients with purging behaviors 1, 2
- Liver enzymes (AST, ALT) to assess hepatic function, which may be severely affected in malnutrition 1, 2, 4
- Renal function tests (BUN, creatinine) to evaluate kidney function and hydration status 1, 2, 5
Additional Testing Based on Clinical Presentation
- Electrocardiogram (ECG) for all patients with restrictive eating disorders, severe purging behaviors, or those taking QTc-prolonging medications to monitor for arrhythmias and sudden cardiac death risk 1, 2
- Thyroid function tests (TSH, free T4) in patients with oligomenorrhea or amenorrhea 2, 6
- 25-hydroxyvitamin D levels if low bone mineral density or bone stress injury is suspected 2
Important Clinical Caveats
Normal Labs Do Not Exclude Serious Illness
Approximately 60% of anorexia nervosa patients show normal laboratory values on routine testing even with severe malnutrition, making clinical assessment paramount 2. The diagnosis of anorexia nervosa remains clinical, based on weight and specific psychopathology, not laboratory findings 6.
Expected Laboratory Abnormalities
When present, laboratory abnormalities in anorexia nervosa typically include 6, 3:
- Hematologic: Normocytic, normochromic anemia; leukopenia (lymphocyte or neutrophil deficiency); thrombocytopenia
- Electrolyte: Hypokalemia, hypochloremia, metabolic alkalosis (especially with purging)
- Hepatic: Elevated transaminases, which may worsen during refeeding 4, 7
- Endocrine: Low T3, low FSH/LH, low estrogen, elevated cortisol 6
Refeeding Considerations
Liver function tests can paradoxically worsen during refeeding due to glucose deposition in hepatocytes or ongoing liver cell death from prolonged starvation 4. Severe cases may show transaminases exceeding 5,000 IU/L with associated hypoglycemia and coagulopathy 7.
Correlation with Clinical Assessment
Laboratory results must be interpreted alongside 1, 2:
- Vital signs: Temperature, resting heart rate, blood pressure, orthostatic pulse and blood pressure changes
- Anthropometrics: Height, weight, BMI (or percent median BMI for adolescents)
- Physical examination: Russell's sign, parotid enlargement, dental erosion, lanugo, acrocyanosis