Calculated Serum Osmolality Can Lead to a Systematic Bias Compared to Direct Measurement

Abstract

The reference method to measure serum osmolality (Mosm) is the delta cryoscopic method. However, the technology may not be available. Clinicians therefore must calculate osmolality (Cosm) as the sum of concentrations of selected principal solutes such as sodium, potassium, urea, and glucose. To evaluate the validity of Cosm in patients in hyperosmolar state we compared Cosm to Mosm. Twenty-two ICU patients treated by infusion of hypertonic solutes for intracranial hypertension following head injury were prospectively studied. A control group of 10 patients with hypernatremia due to medical causes was also evaluated. Na+, blood urea nitrogen (BUN), and glucose were measured to calculate serum osmolality: Cosm = (2 x Na+) + BUN + glucose (in mOsm/kg). Measurement of serum osmolality was performed using the delta-cryoscopic method. The results of the two methods were compared by correlating the difference (Mosm-Cosm) between each pair of results with the mean of the pairs of results. Cosm underestimated Mosm (3.4 mOsm/kg, P < 0.02) in the control group whereas Cosm overestimated Mosm in patients who received hypertonic fluids (2.3 mOsm/kg, P < 0.03). Calculation of osmolality introduced a systematic bias, overestimating osmolality in the lower ranges and underestimating it in the higher ranges. In the range of hyperosmolality commonly used to manage intracranial hypertension following head trauma, serum osmolality, as determined by sum of principal serum solutes, compares poorly with direct measurement using standard osmometry.

Agreement of measured and calculated serum osmolality during the infusion of mannitol or hypertonic saline in patients after craniotomy: a prospective, double-blinded, randomised controlled trial

“…Among these formulas, several have been evaluated in neurological and neurosurgical patients during the administration of hyperosmolar agents [ 22 – 24 ]. In these studies, different formulas were used and different patients were enrolled (not treated with hyperosmolar agents [ 24 ], treated with mannitol [ 22 ] or treated with both mannitol and HS [ 23 ]). The results showed a suboptimal correlation between measured and calculated serum osmolality during mannitol administration [ 22 , 23 ].…”

“…In these studies, different formulas were used and different patients were enrolled (not treated with hyperosmolar agents [ 24 ], treated with mannitol [ 22 ] or treated with both mannitol and HS [ 23 ]). The results showed a suboptimal correlation between measured and calculated serum osmolality during mannitol administration [ 22 , 23 ]. Although patients receiving mannitol and HS were both enrolled in the study of Vialet et al, results of mannitol and HS were not reported separately [ 23 ].…”

“…The results showed a suboptimal correlation between measured and calculated serum osmolality during mannitol administration [ 22 , 23 ]. Although patients receiving mannitol and HS were both enrolled in the study of Vialet et al, results of mannitol and HS were not reported separately [ 23 ]. In present study, we chose 4 most cited formulas [ 21 ], and confirmed the most accurate formula for the estimation of serum osmolality.…”

“…This poor agreement of measured and calculated osmolality during mannitol infusion might be due to the osmolal gap, which is the difference between the two values. Up to now, few studies have been carried out to determine the accuracy of serum osmolality estimation during HS infusion [ 23 ]. In the present study, mannitol or HS was used in patients after elective craniotomy, and serum osmolality was measured during drug infusion.…”

Hypertonic saline solutions for treatment of intracranial hypertension

“…To maximize benefits from hypertonic saline, clinical trials and experts have reported crossing this boundary in everyday care, without complaining about an increase in grave adverse effects such as acute renal failure [18]. During such therapy, laboratory values should be monitored every 3-6 h, and meticulous attention to provide an adequate hydration status is Intracranial hypertension Himmelseher 415 [19]. Calculations consistently overestimated measured osmolalities, which led to the conclusion that direct measurements are indispensable when osmolality is checked with hypertonic saline therapy.…”