Is Lactated Ringer's Solution Isotonic?
Lactated Ringer's solution is technically hypotonic, not isotonic, with an osmolarity of approximately 273-277 mOsm/L, which falls below the isotonic range of 280-310 mOsm/L. 1
Understanding the Tonicity Classification
The classification of LR as hypotonic versus isotonic depends on whether you measure real osmolality versus theoretical osmolality:
- Real osmolality measurement: LR has an osmolarity of 273-277 mOsm/L, making it hypotonic compared to plasma (275-295 mOsm/L) 1
- Isotonic range definition: Solutions are considered isotonic when osmolarity ranges from 280-310 mOsm/L (examples: 0.9% NaCl, Plasma-Lyte, Isofundine) 2
- Normal saline comparison: 0.9% NaCl has an osmolarity of 308 mOsm/L and is truly isotonic 1
Critical Clinical Implications of LR's Hypotonic Nature
Brain Injury - Absolute Contraindication
Lactated Ringer's should be avoided in patients with acute brain injury or traumatic brain injury due to its hypotonic nature and risk of worsening cerebral edema. 2, 1
- A multicenter study in traumatic brain injury patients showed higher mortality in the LR group compared to isotonic 0.9% NaCl (HR 1.78,95% CI 1.04-3.04, p=0.035) 2
- Guidelines specifically recommend isotonic crystalloids (0.9% NaCl) as first-line therapy for acute brain injury patients (GRADE 2+, Strong Agreement) 2
- The mechanism of harm is increased cerebral edema and intracranial hypertension from the hypotonic solution 2
When LR Is Still Appropriate
Despite being hypotonic, LR remains recommended in certain clinical scenarios:
- Burn resuscitation: LR is recommended as first-line balanced fluid for burns victims due to its physiologic electrolyte composition 1
- General resuscitation: LR is acceptable for most non-brain-injured patients requiring volume expansion 1
- Balanced electrolyte profile: The Na⁺:Cl⁻ ratio more closely resembles plasma compared to normal saline 1
Common Pitfall to Avoid
The most critical error is assuming LR is isotonic and using it in brain-injured patients. Many clinicians and reference texts incorrectly classify LR as "isotonic crystalloid" when it is actually hypotonic by real osmolality measurement 2, 1, 3. This terminology confusion occurs in approximately 54% of medical reference books 3.