Most Appropriate Initial Laboratory Test
The most appropriate initial laboratory test is fasting plasma glucose (or fasting insulin/HOMA-IR assessment), as this obese boy with acanthosis nigricans and hypertension has classic clinical features of insulin resistance syndrome, which requires metabolic screening rather than evaluation for rare secondary causes of hypertension.
Clinical Reasoning
Why Metabolic Screening Takes Priority
The clinical presentation strongly suggests insulin resistance syndrome rather than a rare endocrine cause of hypertension:
Acanthosis nigricans is a cutaneous marker of insulin resistance and is specifically listed as a sign warranting screening for insulin resistance and type 2 diabetes in obese children 1
The American Heart Association explicitly recommends fasting plasma glucose testing for children who are overweight AND have signs of insulin resistance such as acanthosis nigricans and hypertension 1
Obesity with acanthosis nigricans creates a high pretest probability for insulin resistance as the underlying cause of hypertension, making metabolic evaluation the logical first step 2, 3, 4
Evidence Supporting Metabolic Testing First
Children with obesity, acanthosis nigricans, hypertension, and dyslipidemia should undergo fasting glucose testing as the initial screening tool for insulin resistance syndrome 1
Obese children with acanthosis nigricans have significantly higher fasting insulin levels, HOMA-IR values, and prevalence of metabolic syndrome compared to obese children without acanthosis nigricans 2, 4
67-72% of obese children with acanthosis nigricans have HOMA-IR >2.5, indicating significant insulin resistance 2
Acanthosis nigricans has a strong association with metabolic syndrome (p=0.003), making it an important clinical marker for identifying at-risk children 3
Why Other Tests Are Not Initial Choices
Plasma catecholamines (Option A):
- Reserved for suspected pheochromocytoma, which presents with paroxysmal hypertension, episodic headaches, palpitations, and sweating 5, 6
- Not indicated without these specific symptoms 5
Plasma renin activity (Option B):
- Used for screening primary aldosteronism or renovascular disease 5, 6
- Primary aldosteronism typically presents with hypokalemia (though most patients are normokalemic), and there's no mention of electrolyte abnormalities here 5
- Renovascular disease is more common in younger children (<6 years) or presents with abrupt onset, severe diastolic hypertension, or abdominal bruit 5, 6
Cortisol (Option C):
- Indicated when clinical features of Cushing syndrome are present: truncal obesity with thin extremities, purple striae, easy bruising, moon facies, buffalo hump 1, 5
- Acanthosis nigricans alone without these specific cushingoid features doesn't warrant cortisol testing 1
Ambulatory BP monitoring (Option D):
- ABPM is recommended to confirm the diagnosis of hypertension before extensive laboratory workup 1, 5, 6
- However, once hypertension is confirmed in an obese child with acanthosis nigricans, metabolic screening takes precedence over evaluation for rare secondary causes 1
- The 2017 AAP guidelines state that children ≥6 years who are overweight/obese with positive family history do NOT require extensive evaluation for secondary causes if there are no specific findings suggesting secondary hypertension 1
Recommended Diagnostic Approach
Initial Laboratory Panel
Based on guideline recommendations for this clinical scenario:
- Fasting plasma glucose (primary screening test) 1
- Fasting insulin and HOMA-IR calculation (provides better assessment of insulin resistance than glucose alone) 1
- Lipid profile (dyslipidemia is part of insulin resistance syndrome) 1, 5
- Basic metabolic panel (electrolytes, creatinine) 1, 5
- Urinalysis (to screen for renal parenchymal disease) 1, 5
Clinical Pitfalls to Avoid
Don't pursue extensive secondary hypertension workup in obese children with acanthosis nigricans without first addressing the obvious metabolic syndrome 1
Normal fasting glucose doesn't exclude insulin resistance - these children remain at high risk for future diabetes and cardiovascular disease even with normal glucose 1
Acanthosis nigricans severity correlates with degree of insulin resistance - its presence should prompt aggressive metabolic screening and lifestyle intervention 2, 3, 4