Laboratory Testing for Excessive Daytime Sleepiness
Order thyroid stimulating hormone (TSH), complete blood count (CBC), comprehensive metabolic panel (CMP) including liver function tests, and ferritin levels as initial laboratory screening for patients with excessive daytime sleepiness. 1, 2
Essential Laboratory Tests
First-Line Blood Work
Thyroid function (TSH): Hypothyroidism is a treatable metabolic cause of hypersomnia that must be excluded 1, 2
Complete blood count (CBC): Identifies anemia and other hematologic conditions contributing to fatigue and sleepiness 1, 2
Comprehensive metabolic panel/serum chemistry: Detects metabolic derangements, renal dysfunction, and electrolyte abnormalities 1, 2
Liver function tests: Hepatic encephalopathy can present as hypersomnia 1, 2
Ferritin level: Critical when restless legs syndrome is suspected (uncomfortable leg sensations worse at night, improved with movement); ferritin <45-50 ng/mL indicates treatable RLS 1
Specialized Testing Beyond Basic Labs
When to Order Sleep Studies
Polysomnography (PSG) followed by Multiple Sleep Latency Test (MSLT) is required when central disorders of hypersomnolence (narcolepsy, idiopathic hypersomnia) are suspected after excluding other causes 1, 2
The MSLT measures sleep latency across 4-5 daytime naps; mean sleep latency ≤8 minutes with ≥2 sleep-onset REM periods confirms narcolepsy 1, 2
PSG is also the gold standard for diagnosing obstructive sleep apnea when patients have snoring, witnessed apneas, or obesity (BMI ≥33 kg/m²) 1, 3
Neuroimaging
Brain MRI should be obtained to identify structural causes including tumors, multiple sclerosis, strokes, intracranial hemorrhage, or neurodegenerative diseases 1, 2
This is particularly important in older adults where hypersomnia may represent early neurodegenerative disease 2
Cerebrospinal Fluid Analysis
CSF hypocretin-1 levels can definitively confirm narcolepsy type 1 (with cataplexy) when levels are ≤110 pg/mL, even without MSLT 1, 2
This test is especially useful when MSLT results are equivocal or cannot be performed 2
Clinical Context for Laboratory Selection
Medication Review First
Before ordering extensive testing, conduct a thorough medication review, as sedating medications (benzodiazepines, opioids, antihistamines, certain antidepressants) are common and often overlooked causes of hypersomnia, particularly in older adults taking multiple medications 2
Risk Stratification
Patients with BMI ≥40 kg/m² or BMI ≥33 kg/m² plus resistant hypertension or type 2 diabetes warrant comprehensive sleep evaluation for obstructive sleep apnea 1
The presence of cataplexy (sudden muscle weakness triggered by emotion), hypnagogic hallucinations, or sleep paralysis alongside sleepiness strongly suggests narcolepsy and necessitates PSG/MSLT 1, 2
Common Pitfalls
Do not assume sleepiness is purely psychiatric (depression) without excluding medical causes through laboratory testing 2
Insufficient sleep syndrome (chronic sleep deprivation from lifestyle) is extremely common and requires sleep diary documentation before pursuing expensive sleep studies 2, 4
In patients with suspected OSA, screen with validated tools (STOP questionnaire, Epworth Sleepiness Scale) before ordering PSG 1