Table of Contents

## How is serum Osmol calculated?

The 1975 Dorwart and Chalmers formula, serum osmolality = 1.86(Na) + (glucose/18) + (BUN/2.8) + 9 had been often used to calculate the plasma osmolality. In 1976, Smithline and Gardner, proposed to use serum osmolality = 2(Na) + glucose/18 + BUN/1.8 as a simpler formula.

**How do you calculate ABG from osmolarity?**

CALCULATION

- Calculated osmolarity = (2 x [Na+]) + [glucose] + [urea])
- Osmolar gap = Osmolality (measured) – Osmolarity (calculated)
- normal = < 10.
- note that is a pragmatic clinical aid – the units are different (osmolality =mOsm/kg and osmolarity = mOsm/L) so it doesn’t make mathematical sense!

**How is urine anion gap calculated?**

Urine anion gap is calculated by subtracting the urine concentration of chloride (anions) from the concentrations of sodium plus potassium (cations): = Na+ + K+ − Cl. where the concentrations are expressed in units of milliequivalents/liter (mEq/L).

### How is water clearance calculated?

By manipulating formula (2), another means of calculating free water clearance follows: 1. CH2O = V(1 − Uosm/Posm) 2.

**How do you calculate urine and serum osmolality?**

A common simplified formula for serum osmolality is: Calculated osmolality = 2 x serum sodium + serum glucose + serum urea (all in mmol/L). Osmolality can also be measure by an osmometer. The difference between the calculated value and measured value is known as the osmolar gap.

**How is serum tonicity calculated?**

Gains in glucose raise the effective extracellular solute and result in hypertonicity. Serum tonicity in patients with hyperglycemia is calculated as the sum of 2×[Na]S plus the serum glucose concentration ([Glu]S) in mmol/L.

#### What increases Osmol gap?

An osmolal gap greater than 10 mOsm/kg indicates the presence of abnormal, unmeasured osmotically active molecules. The most common cause of an elevated osmolar gap is ingestion of an alcohol, including ethanol, methanol, ethylene glycol, diethylene glycol, propylene glycol, and isopropanol (isopropyl alcohol).

**How do you calculate the anion gap?**

Expressed in words, the equation is: Anion Gap = Sodium – (Chloride + Bicarbonate) which is logically equivalent to: Anion Gap = (the most prevalent cation) minus (the sum of the most prevalent anions)

**When do we measure anion gap?**

An anion gap blood test checks the acid-base balance of your blood and if the electrolytes in your blood are properly balanced. Healthcare providers most commonly use anion gap to identify cases of metabolic acidosis — when you have higher-than-normal amounts of acid in your blood.

## What is normal urine anion gap?

For the urine anion gap, the most prominently unmeasured cation is NH4+. Healthy subjects typically have a gap of 0 to slightly normal (< 10 mEq/L). A urine anion gap of more than 20 mEq/L is seen in metabolic acidosis when the kidneys are unable to excrete NH4+ (such as in renal tubular acidosis).

**How is free water clearance calculated?**

**How do you calculate osmolarity and osmolality?**

Osmolarity and Osmolality

- mOsmol/kg = n x mmol/L.
- mOsmol/kg = (n x mg/dL x 10) ÷ mol wt (g)
- mOsmol/kg = (n x mEq/L) ÷ valence.

### What is the difference between urine osmolality and serum osmolality?

The difference between the calculated value and measured value is known as the osmolar gap. Unlike urine osmolality, specific gravity, is affected by both the number and size of particles in solution….Serum and Urine Osmolality.

Conditions that increased osmolality3 | |
---|---|

Serum | Urine |

Serum | Urine |

**How do you calculate plasma tonicity?**

The equation: Posm =2 [Na(+)]+glucose (mg/dL)/18+BUN (mg/dL)//2.8 is also the simplest and best formula to calculate plasma osmolality. The concentration of only effective osmoles evaluates effective osmolality or tonicity as: Eosm =2 [Na(+)]+glucose/18. The normal range of plasma tonicity is 275-295mOsm/kg of water.