MLSCI 262 - Osmolality

What is the anion gap?

The difference between routinely measured cations and anions; estimates unmeasured anions.

What is the formula for the anion gap?

AG = [Na+] - ([Cl-] + [HCO3-]).

What is the lecture reference interval for the anion gap?

4–16 mmol/L.

Why is K+ excluded from the standard anion gap formula?

Its extracellular concentration is relatively low.

Which four electrolytes are routinely measured in the clinical lab?

Na+, K+, Cl-, and HCO3-.

Why do measured plasma cations appear to exceed measured anions?

Not all ions are routinely measured; total positive and negative charges actually balance.

What is the principle of electroneutrality?

Total positive charge equals total negative charge in a solution.

Name unmeasured anions that contribute to the anion gap.

Albumin/proteins, phosphate, sulfate, organic acids such as lactate and ketoacids.

Calculate AG: Na+ 140, Cl- 100, HCO3- 25 mmol/L.

140 - (100 + 25) = 15 mmol/L; within lecture reference interval.

What is the most common general reason for an increased anion gap?

Increased unmeasured anions.

Which organic acids may increase the anion gap?

Lactate, ketoacids (beta-hydroxybutyrate/acetoacetate), formate, oxalate, and salicylate.

Why can renal failure increase the anion gap?

Failure to eliminate metabolic acids, including phosphate and sulfate species.

How can alkalosis increase the anion gap?

Increases the net negative charge on proteins.

How can low calcium or magnesium affect the anion gap?

Decreased unmeasured cations can increase the gap.

Why is a decreased anion gap less common?

It is less likely than an increased gap, according to the lecture.

What changes in unmeasured anions can decrease the anion gap?

Low albumin or low phosphate.

What changes in unmeasured cations can decrease the anion gap?

High calcium or high magnesium.

How can multiple myeloma decrease the anion gap?

Increased positively charged antibodies/proteins act as unmeasured cations.

Calculate AG: Na+ 138, Cl- 105, HCO3- 24 mmol/L.

138 - (105 + 24) = 9 mmol/L; within lecture reference interval.

Calculate AG: Na+ 142, Cl- 99, HCO3- 15 mmol/L.

142 - (99 + 15) = 28 mmol/L; increased.

What collection issues should be avoided for electrolyte testing?

Hemolysis, delayed cell separation, exercise before collection, and fist pumping.

Which anticoagulants may be in green-top tubes for electrolyte testing?

Lithium or ammonium heparin.

Which listed tubes contain no anticoagulant?

Red tiger-striped serum separator, red-top, and gold-top tubes.

Define a mole.

6.022 × 10^23 particles (Avogadro’s number).

Define an osmole.

An amount producing 6.022 × 10^23 osmotically active particles in solution.

How do moles and osmoles differ?

Moles count formula units/molecules; osmoles count dissolved osmotically active particles.

How many osmoles are produced by 1 mol of glucose?

1 Osmol, because glucose does not dissociate.

How many ideal osmoles are produced by 1 mol of NaCl?

2 Osmol: Na+ and Cl-.

How many ideal osmoles are produced by 1 mol of CaCl2?

3 Osmol: one Ca2+ and two Cl-.

What is n in the theoretical osmole calculation?

Number of dissolved particles/ions produced per formula unit.

What is the theoretical formula for osmoles from a dissociating solute?

Osmoles = moles × n.

Why may the theoretical number of ions overestimate osmoles in biological fluids?

Ions do not behave as fully independent particles; the lecture accounts for incomplete dissociation.

What is the Van’t Hoff factor i in the lecture?

Effective particle factor: i = n × Φ, where Φ is the osmotic coefficient.

Calculate ideal osmoles from 0.5 mol of NaCl.

0.5 × 2 = 1 Osmol.

Calculate ideal osmoles from 0.2 mol of CaCl2.

0.2 × 3 = 0.6 Osmol.

Define molarity and give its unit.

Amount of an individual solute per volume; mol/L.

Define molality and give its unit.

Amount of an individual solute per mass of solvent; mol/kg.

Define osmolarity and give its unit.

Total osmotically active particles per volume; Osmol/L.

Define osmolality and give its unit.

Total osmotically active particles per mass of solvent; Osmol/kg.

Which two concentration terms use volume as denominator?

Molarity and osmolarity.

Which two concentration terms use mass as denominator?

Molality and osmolality.

Which terms count individual solute amount rather than total osmotically active particles?

Molarity and molality.

Why is osmolality preferred over osmolarity for consistent measurement?

Mass is unaffected by temperature/pressure, while volume can change.

What is the mass of 1 L of water approximately?

1 kg.

What is the lecture reference interval for serum osmolality?

280–300 mOsm/kg.

What is a solute?

The substance being dissolved.

What is a solvent?

The medium that dissolves a solute.

Define osmosis.

Movement of solvent across a semipermeable membrane according to solute concentration.

In which direction does solvent move during osmosis?

From higher solvent concentration (lower solute) to lower solvent concentration (higher solute).

Define osmotic pressure.

Pressure needed to prevent solvent movement across a semipermeable membrane.

What determines osmotic pressure?

Number/concentration of osmotically active solute particles.

What osmotic pressure equation appears in the lecture?

π = iMRT (i: Van’t Hoff factor; M: molarity; R: gas constant; T: kelvin).

Define osmometry.

Measurement of concentration of osmotically active solute particles.

Which main plasma components appear in calculated osmolality?

Sodium-associated particles, glucose, and urea.

What is the lecture’s calculated osmolality formula?

mOsm/kg = 1.86[Na+] + [glucose] + [urea] + 9 (inputs in mmol/L).

What is the rough calculated osmolality formula?

mOsm/kg ≈ 2[Na+] + [glucose] + [urea] (inputs in mmol/L).

Why is sodium approximately doubled in the rough osmolality formula?

Sodium is accompanied by counterions, which also contribute osmotic particles.

Calculate rough osmolality: Na 140, glucose 5, urea 5 mmol/L.

2(140) + 5 + 5 = 290 mOsm/kg.

Calculate osmolality with full formula: Na 140, glucose 5, urea 5.

1.86(140) + 5 + 5 + 9 = 279.4 mOsm/kg.

What does a freezing-point osmometer measure directly?

Depression of the sample freezing point relative to pure water.

Why does dissolved solute lower water’s freezing point?

Solute particles cause a colligative freezing-point depression; more particles cause a larger decrease.

What is the freezing point of pure water?

0.00°C.

What is the approximate freezing point of normal serum in the lecture?

−0.54°C.

What is the molal freezing point depression constant of water?

1.86°C·kg/mol.

What is Δ in freezing point depression calculations?

Freezing point of pure water minus freezing point of patient sample.

What formula converts freezing point depression to osmolality?

Osmolality (mOsm/kg) = (Δ / 1.86) × 1000.

Calculate osmolality if a sample freezes at −0.558°C.

(0.558 / 1.86) × 1000 = 300 mOsm/kg.

Calculate osmolality if a sample freezes at −0.465°C.

(0.465 / 1.86) × 1000 = 250 mOsm/kg.

How does greater solute particle concentration affect freezing point?

It causes a larger freezing point depression (more negative freezing temperature).

What is a limitation of calculated osmolality?

It does not capture all unmeasured uncharged solutes.

Define the osmolal gap.

Measured osmolality minus calculated osmolality.

What is the lecture reference range for the osmolal gap?

≤10 mOsm/kg.

What can an increased osmolal gap indicate?

Unmeasured osmotically active compounds, including alcohols, drugs, and metabolic by-products.

Which alcohols are highlighted as possible osmolal gap contributors?

Ethanol, methanol, and ethylene glycol.

What metabolic by-product is highlighted as contributing to the osmolal gap?

Acetone.

Calculate osmolal gap: measured 320, calculated 290 mOsm/kg.

320 - 290 = 30 mOsm/kg; increased.

Calculate osmolal gap: measured 295, calculated 289 mOsm/kg.

295 - 289 = 6 mOsm/kg; within lecture reference range.

How do anion gap and osmolal gap differ?

Anion gap estimates unmeasured charged ions; osmolal gap detects additional osmotically active particles, often uncharged.

What is specific gravity (SG)?

A measure of solute concentration based on density relative to water.

What is density?

Mass per unit volume, e.g., g/mL.

What is the specific gravity formula for urine?

SG = density of urine / density of an equal volume of water.

What two solute properties influence specific gravity?

Number of solutes and their molecular weights.

Does specific gravity have units?

No; it is a ratio of densities.

What urine SG range is listed in the lecture?

1.002–1.040; average approximately 1.017 ± 0.007.

What three methods are listed for urine SG testing?

Urinometer, specific gravity dipstick, and refractive index/refractometer.

What does the urine SG dipstick primarily respond to?

Urine ionic concentration (ionic strength), via a polyelectrolyte reaction.

What chemical principle underlies the urine SG dipstick?

Urine ions change dissociation/pKa behavior of pretreated polyelectrolytes in the pad.

What happens to polyelectrolyte acid groups as urine ionic concentration increases?

More groups dissociate, releasing more H+.

How does increased urine ionic concentration change the dipstick pad pH?

It makes the reagent pad more acidic.

Which pH indicator is in the urine SG dipstick pad?

Bromothymol blue.

How does the urine SG dipstick produce a readable result?

H+ released from polyelectrolytes changes pad pH; bromothymol blue changes color.

What SG increments and range are listed for the dipstick?

Increments of 0.005, from 1.000 to 1.030.

How does increasing urine SG affect the dipstick indicator color?

The slide chart shifts from blue/green at low SG toward yellow at high SG.

What is the principle of refractometric SG measurement?

Solute-dependent bending of light (refractive index) provides an indirect SG estimate.

What happens to refracted light at higher solute concentrations?

It bends more toward the normal.

What is the “normal” in refraction diagrams?

An imaginary line perpendicular to the boundary between media.

What law relates refractive indices and light angles?

Snell’s law: N1 sinθ1 = N2 sinθ2.

How do dipstick and refractometer methods differ?

Dipstick responds mainly to ionic strength; refractometer responds to refractive index affected by solute concentration.

Why can SG and osmolality respond differently to solute composition?

SG depends on density and molecular weight; osmolality depends on number of osmotically active particles.