What laboratory tests are appropriate for a teenager with chronic orthostatic intolerance?

Laboratory Tests for Teenagers with Chronic Orthostatic Intolerance

For teenagers with chronic orthostatic intolerance, appropriate laboratory tests should include electrolytes, renal function, complete blood count, and serum tryptase, particularly when evaluating for underlying causes and comorbid conditions. 1, 2

Initial Laboratory Evaluation

• Basic metabolic panel:

  • Sodium, potassium, magnesium levels (especially important for patients who may start medications like fludrocortisone) 2
  • Renal function tests (BUN, creatinine)
  • Glucose (to rule out hypoglycemia as a cause of symptoms)

• Complete blood count (CBC):

  • To assess for anemia, which can exacerbate orthostatic symptoms 1
  • To evaluate for signs of infection or inflammation

• Thyroid function tests:

  • TSH, free T4 to rule out thyroid dysfunction that can mimic or exacerbate orthostatic symptoms

Additional Testing Based on Clinical Suspicion

• Serum tryptase:

  • Particularly important if mast cell activation syndrome (MCAS) is suspected
  • Should be measured at baseline and during symptom flares (1-4 hours after onset) 2
  • Approximately 25% of MCAS patients have concurrent POTS

• Autoimmune markers:

  • Consider ANA, rheumatoid factor, and other autoimmune markers
  • Relevant due to increased prevalence of autoimmune disorders in patients with orthostatic intolerance 2

• Catecholamine levels:

  • Plasma norepinephrine levels while supine and standing
  • May help differentiate between different forms of orthostatic intolerance 3
  • Elevated standing norepinephrine (>600 pg/mL) may support diagnosis of hyperadrenergic POTS

Cardiovascular Assessment

• ECG:
  • To rule out cardiac causes of symptoms
  • To assess for QT interval abnormalities or other arrhythmia patterns 1

Diagnostic Testing Beyond Laboratory Work

While not laboratory tests per se, these diagnostic assessments are crucial for evaluation:

• Active standing test:

  • Measurement of BP and heart rate supine and during active standing for 3 minutes 1
  • Diagnostic when there is a symptomatic fall in systolic BP ≥20 mmHg or diastolic BP ≥10 mmHg; or decrease in systolic BP to <90 mmHg
  • In teenagers, a heart rate increase ≥40 bpm within 10 minutes of standing is diagnostic for POTS 1, 2

• Tilt table testing:

  • Particularly useful when initial active standing test is inconclusive
  • Optimal diagnostic heart rate increase cutoff of 38 bpm 2

Clinical Pearls and Pitfalls

• Timing matters: Some laboratory abnormalities may only be present during symptomatic episodes (e.g., tryptase in MCAS)
• Consider comorbidities: Teenagers with orthostatic intolerance often have gastrointestinal symptoms, chronic fatigue, or hypermobile Ehlers-Danlos syndrome that may require additional testing 2, 4, 5
• Follow-up testing: The American College of Cardiology recommends monitoring electrolytes and renal function every 3-6 months, especially for patients on medications like fludrocortisone 2
• Avoid overdiagnosis: Not all orthostatic symptoms require extensive laboratory evaluation; focus testing on clinically suspected conditions

Monitoring

• For patients started on treatment, follow-up laboratory testing should be performed:
  • Every 3-6 months
  • When changing treatment regimens
  • During significant symptom exacerbations
  • When starting new medications that may affect electrolyte balance 2

Remember that orthostatic intolerance in teenagers can significantly impact quality of life, including physical, social, and academic functioning 6. Appropriate laboratory evaluation helps identify underlying causes and guides effective treatment strategies.