Use of the advanced micro‐osmometer model 3300 for determination of a normal osmolality and evaluation of different formulas for calculated osmolarity and osmole gap in adult dogs

Abstract

Objective: To determine a reference interval of whole blood and plasma osmolalities for dogs using the Advanced Micro Osmometer Model 3300, to compare calculated osmolarity to measured osmolality, to determine a reference osmole gap, and to determine the best formula for calculated osmolaity. Design: Prospective, observational. Setting: Tertiary referral and teaching hospital. Animals: One hundred healthy adult dogs. Interventions: None. Measurements: Serum and whole blood biochemistry and osmolality assessments. Results: The mean and median of the measured whole blood osmolality were 323 and 320 mOsm/kg, respectively, with a standard deviation of 13.2 mOsm/kg. The mean and median of the measured plasma osmolality were 313 and 310 mOsm/kg, respectively, with a standard deviation of 13.2 mOsm/kg. The formula that was closest to predicting the measured whole blood and plasma osmolality was ((1.86(Na+K))+(BUN/2.8)+(Glucose/18))/0.93 followed closely by the traditional formula of (2(Na+K))+(BUN/2.8)+(Glucose/18). The mean calculated osmolarities using these formulas were 314.1 and 313.25 mOsm/L, respectively. The mean osmole gap using these formulas was 3.49 and 4.41 mOsm, respectively, for whole blood and −2.01 and −1.1 mOsm, respectively, for plasma. Conclusion: The Advanced Micro Osmometer Model 3300 was successful in measuring the osmolality in relative agreement with the current published reference intervals for osmolality. Measured osmolality correlated well with traditional calculated osmolarity.

Colloid Osmotic Pressure and Osmolality Monitoring

“…10,12 Freezing point depression is the more common and accurate method because it measures volatile substances in solution (e.g., alcohol) that would be missed using the other method. 11 For every 1 mol of nondissociating molecules dissolved in 1 kg (or 1 L) of water, the freezing point depression is decreased by 1.86°C. The osmolality of this solution would be 1 Osm/kg or 1000 mOsm/kg.…”

Osmolality and Colloid Osmotic Pressure

“…Four equations are reported to be used in the calculation of osmolality. 6 The two that correlate in the Starling equation. 5 One major force that opposes hydrostatic pressure across the vascular membrane and thus prevents great movements of water out of the vessel and into the interstitium is plasma colloid osmotic pressure (COP).…”

“…There is a normal 10 mOsm difference between the calculated and measured milliosmoles (osmolar gap). 6 If the osmolar gap is significantly increased, toxins should be investigated for (Table 51.1).…”

“…The Advanced Micro Osmometer Model 3300 (Advanced Instruments, Inc., Norwood, MA), a freezing point depression osmometer, has been validated for measuring plasma and whole blood samples in normal dogs. 6 Osmometers can test a variety of fluid and tissue samples, and plasma, serum, blood, and urine are the most commonly tested samples in veterinary medicine. Serum osmolality can be compared with urine osmolality to evaluate for the cause of water imbalances.…”

“…9 Whole blood osmolality is a little higher than plasma osmolality. 6 Normal urine osmolality can range between 161 and 2830 mOsm in dogs. 10 The urine osmolality will increase when water intake is withheld as long as kidney function is normal.…”