Is Half Normal Saline (0.45% NaCl) Hypotonic?
Yes, half normal saline (0.45% sodium chloride) is definitively a hypotonic solution.
Definition and Classification
Half normal saline (0.45% NaCl) contains 77 mEq/L of sodium, which is significantly lower than the normal plasma osmolality (approximately 280-295 mOsm/kg). This makes it hypotonic relative to plasma. The clinical evidence consistently refers to 0.45% saline as "hypotonic" when compared to isotonic solutions like 0.9% normal saline (154 mEq/L sodium) 1, 2, 3.
Clinical Significance
The hypotonic nature of 0.45% saline carries important clinical implications, particularly regarding the risk of iatrogenic hyponatremia:
Multiple randomized controlled trials demonstrate that hypotonic fluids (including 0.45% saline) increase the risk of hyponatremia compared to isotonic solutions in hospitalized children 1, 3
In term newborns, 0.45% saline can lead to unsafe plasma sodium decreases (>0.5 mEq/L/h), with an 8-fold increased risk compared to isotonic fluids 2
The fall in serum sodium is particularly significant at 12 and 24 hours when using hypotonic maintenance fluids 3
Current Guideline Recommendations
Recent guidelines favor isotonic solutions over hypotonic fluids for maintenance therapy. The ESPNIC 2022 guidelines systematically reviewed multiple RCTs comparing isotonic versus hypotonic maintenance fluids, consistently showing lower risk of hyponatremia with isotonic solutions 1. The NICE 2015 guidelines similarly recommend isotonic fluids as maintenance therapy in term newborns 2.
In summary: 0.45% NaCl is hypotonic by definition and clinical classification, containing half the sodium concentration of isotonic 0.9% saline, which translates to measurable clinical risks when used for maintenance fluid therapy.