Lena Savoie
The osmolal gap is the difference between measured serum osmolality and calculated serum osmolality. It can be used to screen for alcohol intoxication and ingestion of other toxins. The osmolal gap increases with increasing ethanol levels and can be used to calculate the contribution of ethanol to serum osmolarity. The formula for this calculation is Osmolal gap = (Ethanol [mg/dL])/3.7 – 0.35 or, in SI units: Osmolal gap (mOsm/kg) = 1.25 (Ethanol [mmol/L]) – 0.35. This formula was derived from a linear regression analysis and was validated on 128 patients.
| Characteristics | Values |
|---|---|
| Calculating osmolality | The osmolality of serum is a measure of the different solutes in plasma. |
| Use cases | Serum osmolality is used to evaluate hyponatremia and screen for alcohol intoxication. |
| Indicators of alcohol intoxication | A significant disparity (>10) between measured and calculated serum osmolality may indicate alcohol ingestion. |
| Indicators of toxic alcohol ingestion | An osmolal gap of greater than 10 may indicate the presence of toxic alcohols such as methanol, ethylene glycol, or isopropyl alcohol. |
| Formula for calculated osmolality | CO = 2 x NA + 1.15 * GLU/18 + BUN/2.8 |
| Formula for osmolar gap | OG = MO - CO |
| Formula for blood alcohol level (BAL) | BAL = osmolar gap x 4.6 |
| Formula for contribution of ethanol to osmolal gap | Osmolal gap = (Ethanol [mg/dL])/3.7 - 0.35 |
| Simplified formula for osmolarity | 2.0×Na + 1.2×Urea + 1.4×Glucose + 1.2×Ethanol |
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What You'll Learn
The relationship between ethanol and osmolal gap
The osmol gap or osmolal gap is an important clinical tool used in the evaluation of suspected toxic alcohol poisoning. Poisonings with toxic alcohols can be quickly recognised by combining the OG with blood pH and AG. The presence of low blood pH, elevated AG, and a greatly elevated OG (>15) is a medical emergency. The osmol gap is altered by all alcohols, including ethanol.
The serum osmolal gap attributed solely to ethanol is calculated based on the difference between measured serum osmolality before and after the addition of ethanol. The serum ethanol concentration in mg/dL is then divided by the serum osmolal gap attributed to ethanol for that sample to determine the conversion factor of ethanol from mg/dL to mOsm/kg H2O. The true average conversion factor of ethanol is 4.6 ± 0.16.
Several studies have suggested that ethanol does not behave ideally in solution and that the constant k may differ significantly from the ideal. The contribution of ethanol to serum osmolality (k) has been calculated to be 4.25. This indicates that ethanol contributes more to total serum osmolality than would be predicted for an ideal solute.
The formula for the relationship between ethanol and the osmolal gap is: Osmolal Gap (mOsm/kg H2O) = 0.23 (Ethanol [mg/dL]) – 1.43. This formula was derived using linear regression analysis relating the osmolal gap due to ethanol and ethanol concentration.
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Calculating ethanol contribution to osmolality
The presence of ethanol in the body can be a common cause of an elevated osmolar gap. This is due to the metabolites of ethanol, even when the ethanol itself is no longer present. Therefore, it is important to determine the relationship between osmolal gap and ethanol level.
To calculate the contribution of ethanol to the osmolal gap, the following formula can be used:
> Osmolal gap = (Ethanol [mg/dL])/3.7 - 0.35
Or, in SI units:
> Osmolal gap (mOsm/kg) = 1.25 (Ethanol [mmol/L]) - 0.35
This formula was derived through linear regression analysis and was validated on 128 patients. The mean residual osmolal gap for this group was 0.84 mOsm/L.
The University of Iowa Hospitals and Clinics use the following formula for calculated osmolality, which includes an additional term for ethanol (EtOH):
> CO/EtoH = 2 x NA + 1.15 * GLU/18 + BUN/2.8 + [EtOH]/3.8
This formula can be used to determine the osmolar gap including the contribution from ethanol.
In summary, the contribution of ethanol to osmolality can be calculated using the above formulas, which take into account the concentration of ethanol in the body and the resulting osmolal gap.
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Using osmolality to screen for alcohol intoxication
Osmolality is a measure of the total number of particles in 1 litre of solution, while serum osmolality is measured by means of vapour pressure or freezing-point depression. Osmolality is determined mainly by sodium (Na), its counter ions, and uncharged species such as glucose (GLU) and urea (UN).
Osmolar gap (OG) is the difference between measured osmolality (MO) and calculated osmolality (CO). A large positive osmolar gap (>15) can indicate the presence of substances such as ethanol, methanol, and isopropanol in the plasma. Ethanol ingestion is a common cause of an elevated osmolar gap.
Significant ethanol intoxication can be detected by measuring a large osmolar gap, i.e., a large difference between the measured and calculated plasma osmolality. This can be done by using the formula: Osmolal gap = (Ethanol [mg/dL])/3.7 – 0.35 or, in SI units: Osmolal gap (mOsm/kg) = 1.25 (Ethanol [mmol/L]) – 0.35. The University of Iowa Hospitals and Clinics use the following formula for plasma samples: CO/EtoH = 2 x NA + 1.15 * GLU/18 + BUN/2.8 + [EtOH]/3.8.
When interpreting the results, it is important to note that substances other than ethanol may occasionally cause a raised osmolar gap. Additionally, the presence of metabolites of ethanol, such as acetaldehyde, can also contribute to an elevated osmolar gap even when the ethanol itself is no longer present in significant amounts. Therefore, while osmolality calculations can be a rapid and effective screening method for ethanol intoxication, it should not be solely relied upon and other clinical factors and assays should also be considered for a comprehensive evaluation.
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The effect of ethanol ingestion on osmolality
Ethanol ingestion can significantly impact osmolality, a measure of the total number of particles in a solution. This impact is particularly notable in cases of heavy ethanol consumption, which can lead to an elevated osmolar gap even 24 hours after ingestion. The osmolar gap is the difference between measured osmolality (MO) and calculated osmolality (CO).
The relationship between ethanol ingestion and increased osmolar gap is well-established, with ethanol being a common cause of elevated osmolar gap in hospital patients. This effect is mainly attributed to the metabolites of ethanol, such as acetaldehyde, which remain in the body even after the ethanol itself is mostly eliminated. The increased osmotic activity of ethanol is suggested to be due to its binding to water molecules.
The specific formula to calculate the contribution of ethanol to the osmolal gap has been a subject of research. One formula suggests that the osmolal gap is equal to the ethanol concentration in mg/dL divided by 3.7. Another study proposed a conversion factor of 4.6, where the osmolal concentration of ethanol is determined by dividing the serum ethanol level in mg/dL by this factor.
The impact of ethanol on osmolality has been studied in animal models as well. Experiments in rats and rabbits showed that administration of ethanol led to a decrease in serum sodium, potassium, calcium, chloride, and magnesium. These findings provide further evidence of the influence of ethanol on osmolality and related physiological processes.
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The osmolal gap equation
The calculated osmolality (CO) is determined using the formula CO = 2 x NA + 1.15 x GLU/18 + BUN/2.8, where NA is sodium, GLU is glucose, and BUN is urea. The osmolar gap can be used to identify the presence of substances such as ethanol, methanol, isopropanol, and ethylene glycol in plasma.
To include the contribution of ethanol to the osmolal gap, the formula becomes CO/EtoH = 2 x NA + 1.15 x GLU/18 + BUN/2.8 + [EtOH]/3.8, where [EtOH] is the concentration of ethanol. The "unexplained" osmolar gap can then be calculated by subtracting the corrected calculated osmolality from the measured osmolality: MO - CO/EtoH.
The relationship between ethanol and the osmolal gap has been studied, and it has been found that the residual osmolal gap increases with increasing ethanol levels. Linear regression analysis has been used to derive a formula for this relationship: Osmolal gap = (Ethanol [mg/dL])/3.7 - 0.35 or, in SI units, Osmolal gap (mOsm/kg) = 1.25 (Ethanol [mmol/L]) - 0.35.
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Frequently asked questions
Osmolality is a measure of the different solutes in plasma. It is used to evaluate the etiology of hyponatremia and can be used to screen for alcohol intoxication by means of the osmolal gap.
The formula for calculating the contribution of alcohol level to osmolality is: Osmolal gap = (Ethanol [mg/dL])/3.7 – 0.35. This formula has been derived through linear regression analysis and has been validated on a group of patients.
The osmolar gap refers to the difference between measured serum osmolality and calculated serum osmolality. An osmolar gap of greater than 10 is considered abnormal and may indicate the presence of toxic alcohols such as methanol, isopropyl alcohol, and ethylene glycol.
The osmolar gap can be calculated by subtracting the calculated serum osmolality excluding ethanol from the measured serum osmolality. The predicted osmolality excluding ethanol can be calculated using the following formula: 2 x Na (mEq/L) + (Urea [mg/dL])/2.8 + (Glucose [mg/dL])/18.
