collegative properties

Colligative Properties Definition

Properties of solutions that depend on the concentration of solute molecules, but not the identity of the solute (includes freezing point depression, boiling point elevation, vapor pressure lowering, osmotic pressure)

Most Common/Gold-Standard Osmometer

Freezing point depression osmometer (chosen for simplicity, speed, small sample volume, and low interferences)

Freezing Point Depression Osmometer Principle

Thermostatically controlled cooling bath super-cools sample, rapid stir mechanism initiates crystallization, temperature probe measures heat of fusion plateau

Boiling Point Elevation in Clinical Samples

Not useful clinically because proteins coagulate and volatile substances (ethanol, methanol, isopropanol) cannot be detected

Vapor Pressure Osmometry Principle

Measures the decrease in dew point temperature caused by the lowered vapor pressure of a solution compared to pure water reference

Main Limitation of Vapor Pressure Osmometry

Cannot detect volatile substances (methanol, ethanol, isopropanol) because they raise vapor pressure, invalidating osmolal gap measurements

Calculated Plasma Osmolality Formula

2(Na+ [mmol/L]) + (glucose [mg/dL] / 18) + (BUN [mg/dL] / 2.8)

Major Plasma Osmotic Substances

Na+, K+, HCO3-, glucose, and urea (Na+, Cl-, and HCO3- contribute ~92% of plasma osmolality)

Normal Plasma Osmolality Reference Range

275–295 mOsm/kg

Osmolal Gap Formula & Normal Value

Measured Osmolality minus Calculated Osmolality. Normally should be zero; a gap >=10 mOsm/kg suggests exogenous substances

Mnemonic for Agents That Increase Osmolal Gap (ME DIE)

Methanol, Ethylene glycol, Diuretics (mannitol), Isopropyl alcohol, Ethanol

Fractional Excretion of Sodium (FENa) Utility

Evaluates kidney failure with low urine output to differentiate pre-renal causes from intrinsic renal failure

FENa Interpretation (

Specific Gravity vs Osmolality in Urine

Specific gravity measures number AND weight of solutes (high MW solutes raise it disproportionately), while osmolality measures ONLY the number of particles

Prerenal Azotemia vs Acute Tubular Necrosis Urine Osmolality

Prerenal AKI shows >400 mOsm/kg (increased water reabsorption); ATN shows <400 mOsm/kg (injured tubules cannot reabsorb water)

Central DI vs Nephrogenic DI Urine Osmolality

Central Diabetes Insipidus shows <=100 mOsm/kg (lack of ADH); Nephrogenic DI shows <300 mOsm/kg (renal ADH resistance)

Normal Urine-to-Serum Osmolality Ratio

1.0 to 3.0 (normal random range); dehydration should ideally concentrate urine to 3-4x plasma osmolality