Serum Osmolality & Osmolal Gap Calculator
Calculate serum osmolality from sodium, glucose, and BUN, then derive the osmolal gap to screen for unmeasured osmoles.
측정 삼투압은 검사실에서 빙점강하법으로 잰 값입니다. 측정값이 없으면 0으로 두면 계산 삼투압만 표시됩니다. 요소질소는 검사지에 BUN 또는 혈중요소질소로 표기됩니다.
정상 참고범위 285~295 mOsm/kg · 에탄올 제외 기준
정상 −14 ~ +10 mOsm/kg · 292 − 290.0
계산 과정
계산 삼투압 해석 — 정상 범위
계산 삼투압이 정상 참고범위(285~295 mOsm/kg) 안에 있습니다. 다만 삼투압이 정상이어도 삼투압차가 벌어져 있으면 미측정 삼투질이 존재할 수 있으므로, 측정 삼투압이 있다면 함께 확인하세요.
삼투압차 해석 — 정상 범위
삼투압차가 정상 범위(−14 ~ +10 mOsm/kg)입니다. 다만 삼투압차가 정상이라고 해서 독성 알코올 섭취를 배제할 수는 없습니다. 섭취 후 시간이 지나 독성 알코올이 이미 산으로 대사된 뒤라면 삼투압차는 정상이고 음이온차만 올라갑니다.
삼투압차 구간
결과 공유
의료 주의사항
- 이 계산기는 참고용이며 진단이나 의료진의 판단을 대체할 수 없습니다. 삼투압 평가는 전해질, 혈액가스, 음이온차, 복용·섭취력을 함께 보고 해야 합니다.
- 삼투압차가 정상이어도 독성 알코올 섭취를 배제하지 못합니다. 메탄올·에틸렌글리콜은 섭취 후 시간이 지나면 산으로 대사되어 삼투압차는 정상으로 돌아오고 음이온차만 올라갑니다. 반대로 섭취 직후에는 삼투압차만 오르고 산증이 없을 수 있습니다.
- 삼투압차의 정상 범위는 개인차가 커서(건강인에서도 −14 ~ +10 분포) 단일 컷오프로 판단하기 어렵습니다. 본인 검사지의 참고치를 확인하세요.
- 계산식에는 여러 변형(1.86 × Na 사용 등)이 있어 검사실마다 계산값이 1~3 mOsm/kg 차이날 수 있습니다. 이 계산기는 가장 널리 쓰이는 2 × Na 식을 사용합니다.
- 의식 저하, 시야 이상, 깊고 빠른 호흡, 알코올·부동액 섭취가 의심되면 계산 결과와 무관하게 즉시 응급실을 방문하세요.
Overview
Serum osmolality is the total concentration of osmotically active particles per kilogram of serum water (mOsm/kg), and it governs how water distributes across body compartments. Labs can measure it directly by freezing point depression, but sodium, glucose, and blood urea nitrogen alone reproduce it closely through the formula 2 × Na + glucose ÷ 18 + BUN ÷ 2.8. This calculator compares the calculated value against the 285–295 mOsm/kg reference interval to flag hyper- or hypo-osmolality, and when a measured value is supplied it derives the osmolal gap between the two. A widened gap means osmoles the formula does not account for — ethanol, methanol, ethylene glycol, mannitol — are circulating, which is why emergency departments rely on it to screen for toxic alcohol ingestion.
Formula
Calculated osmolality (mOsm/kg) = 2 × Na⁺ (mEq/L) + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8 | SI units = 2 × Na⁺ (mmol/L) + glucose (mmol/L) + urea (mmol/L) | Ethanol correction = add ethanol (mg/dL) ÷ 3.7 | Osmolal gap = measured − calculated | Normal: osmolality 285–295 mOsm/kg, osmolal gap −14 to +10 mOsm/kg
How to Use
- 1Enter sodium in mEq/L. A panel reporting mmol/L gives the same number because sodium is monovalent.
- 2Enter glucose and BUN in mg/dL, or switch the unit selector to SI and enter glucose and urea in mmol/L.
- 3If the lab measured osmolality by freezing point depression, enter it; leave it at 0 to see the calculated value alone.
- 4Enter a serum ethanol level when one is available so the gap can be recomputed with the ethanol contribution removed.
- 5Read the osmolality classification and the osmolal gap band with its interpretation.
Examples
Healthy adult — calculated 290, gap 2
A typical normal panel. Calculated and measured values sit almost on top of each other, so no unmeasured osmoles are present.
Diabetic ketoacidosis — glucose drives osmolality to 306.8
A patient with glucose of 650 mg/dL. Hyperglycaemia pulls water into the vasculature so sodium reads low, yet total osmolality is high.
Suspected methanol — gap of 36.2
The calculated osmolality of 293.8 looks reassuring, but the measured value of 330 opens a 36.2 gap that calculation alone would have missed.
Intoxicated patient — gap falls from 55.2 to 5.2 after ethanol correction
Serum ethanol of 185 mg/dL. The uncorrected gap looks alarming, but accounting for ethanol closes it completely.
Background
Each coefficient comes from the molecular weight of its solute. Sodium is doubled to count the chloride and bicarbonate that accompany it, glucose is divided by 18 because its molecular weight of 180 converts mg/dL to mmol/L after the decilitre-to-litre factor, and BUN is divided by 2.8 because urea nitrogen carries two nitrogen atoms (14 × 2 = 28). Ethanol has a molecular weight of 46, yet 3.7 rather than 4.6 is used by convention because it agrees better with measured osmolality. One prerequisite matters for the gap: the measured value must come from freezing point depression. Vapour pressure osmometers do not detect volatile alcohols, so a toxic ingestion leaves the gap deceptively normal. Several formula variants also exist, such as 1.86 × Na + glucose ÷ 18 + BUN ÷ 2.8 + 9, which is why calculated values can differ by 1–3 mOsm/kg between labs.
Tips
- ✔The gap is measured minus calculated. Reversing the order flips the sign and inverts the interpretation.
- ✔A normal gap does not rule out toxic alcohol ingestion — once the alcohol is metabolised to acid, the gap normalises and only the anion gap stays elevated.
- ✔Early after ingestion the opposite holds: the gap is wide while acidosis has not yet developed, so pair this with an anion gap calculation.
- ✔With marked hyperglycaemia, also consider effective osmolality (2 × Na + glucose ÷ 18, excluding urea), because urea crosses cell membranes freely and does not drive water shifts.
- ✔Severe hypertriglyceridaemia or hyperproteinaemia lowers measured sodium (pseudohyponatraemia), depressing only the calculated value and opening a spurious gap.
- ✔Mannitol, glycerol, and the propylene glycol vehicle in infused lorazepam or phenytoin all widen the gap, so review the medication record before pursuing a toxicology workup.
FAQ
Q. Is osmolality the same as osmolarity?
Osmolality is particles per kilogram of solvent (mOsm/kg) and osmolarity is particles per litre of solution (mOsm/L). Plasma is roughly 93% water, so the two are numerically close and the terms are used interchangeably in practice, but laboratories measure osmolality and the formula is conventionally reported in mOsm/kg.
Q. Is the calculator useful without a measured value?
Yes. The calculated value alone screens for hyperosmolality from dehydration, hyperglycaemia, or uraemia and for hypo-osmolality from hyponatraemia or water excess. The osmolal gap, however, requires a measured result, and toxic alcohol screening specifically requires freezing point depression.
Q. My gap is just over 10 — does that mean poisoning?
No. Healthy people scatter anywhere from −14 to +10, so 10 to 20 is a grey zone. Ethanol, mannitol or glycerol infusions, propylene glycol vehicles, and simple lab imprecision are the usual explanations. Suspicion for toxic alcohol rises when the gap clears 20 and a high anion gap metabolic acidosis accompanies it.
Q. How does this differ from the anion gap?
The anion gap examines charge balance and detects unmeasured anions such as lactate and ketoacids, while the osmolal gap counts particles and detects unmeasured osmoles such as alcohols and mannitol. Methanol and ethylene glycol raise only the osmolal gap early and only the anion gap later, so checking both avoids missing the ingestion regardless of timing.
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