In a 60-year-old woman with nocturnal aching in both upper and lower limbs consistent with restless legs syndrome, does a serum ferritin level of 316 ng/mL rule out the diagnosis?

Ferritin Level of 316 ng/mL Does NOT Rule Out Restless Legs Syndrome

A ferritin level of 316 ng/mL does not exclude the diagnosis of restless legs syndrome (RLS), and this patient may still benefit from intravenous iron therapy despite having a "normal" ferritin level. Recent evidence demonstrates that patients with RLS and ferritin levels between 100-300 µg/L show significant clinical improvement with IV iron treatment 1.

Why This Ferritin Level Doesn't Rule Out RLS

Brain Iron Deficiency Can Exist Despite Normal Serum Ferritin

• RLS is fundamentally a disorder of brain iron deficiency, particularly in the substantia nigra, which may not correlate with serum ferritin levels 1
• Patients with ferritin levels up to 300 ng/mL have demonstrated:
  • Significant improvement in International RLS Severity Scale scores after IV iron
  • Increased substantia nigra iron deposits (measured by echogenicity index)
  • Similar therapeutic response to patients with ferritin <100 ng/mL 1

The Evidence Challenges Traditional Cutoffs

The 2025 study by Garcia-Malo et al. directly challenges current guidelines by showing that IV iron (1,000 mg ferric carboxymaltose) is effective and safe in patients with ferritin 100-300 µg/L 1. Both groups (ferritin <100 vs. 100-300) showed comparable improvements without significant risk of iron overload—only 2 patients developed transferrin saturation >45%, with no hepatic dysfunction 1.

Clinical Approach for This Patient

Confirm the Diagnosis First

• Verify all four International RLS Study Group criteria are met:
  • Urge to move legs with uncomfortable sensations
  • Symptoms worse at rest/inactivity
  • Relief with movement
  • Circadian pattern (worse evening/night)
• Document symptom frequency (≥5 times per month for clinical significance) 2

Check Additional Iron Parameters

Critical caveat: Ferritin levels vary significantly by assay method. The Roche method reads approximately 1.5x higher than Beckman for the same sample 3. If your lab uses Roche, a ferritin of 316 ng/mL may be equivalent to ~200 ng/mL on Beckman 3.

Obtain:

• Transferrin saturation (more reliable than ferritin alone)
• Serum transferrin levels
• Consider which assay your lab uses 3

Treatment Algorithm

For this patient with ferritin 316 ng/mL:

1. If transferrin saturation <20%: Consider IV iron therapy despite the elevated ferritin 1

   • Use ferric carboxymaltose 1,000 mg
   • Target ferritin >100 ng/mL (though this patient already exceeds this)
   • Monitor for transferrin saturation >45% post-treatment

2. If iron parameters are truly replete: Start gabapentin or pregabalin as first-line pharmacotherapy 4, 5

   • These are now preferred over dopamine agonists in 2025 guidelines
   • Dopamine agonists carry significant augmentation risk and should only be used short-term at low doses 4, 5

3. Avoid dopamine agonists as standard therapy: The 2025 AASM guidelines suggest against routine use of pramipexole, ropinirole, and rotigotine due to augmentation risk 4

Important Pitfalls to Avoid

• Don't dismiss RLS based solely on "normal" ferritin: The association between serum ferritin and RLS is complex, and some patients with high ferritin still have RLS 6
• Don't assume all ferritin assays are equivalent: A ferritin of 316 ng/mL on Roche may require different interpretation than on Beckman 3
• Don't start dopamine agonists first-line: This was standard in 2012 but is now explicitly discouraged due to augmentation risk 4, 5
• Consider IV iron even with "adequate" ferritin: The 2025 data supports treatment up to ferritin 300 µg/L 1

Special Consideration for ESRD Patients

If this patient had end-stage renal disease, the AASM specifically recommends IV iron sucrose for ferritin <200 ng/mL and transferrin saturation <20% 4. This reinforces that higher ferritin thresholds may be appropriate in RLS.