{
  "slug": "osmolar-gap",
  "title": "Osmolar Gap Calculator",
  "heading": "Serum Osmolar Gap Calculator",
  "category": "fitness-health",
  "url": "https://www.revenuelab.fyi/toolbox/osmolar-gap",
  "summary": "Screen for unmeasured osmoles like toxic alcohols.",
  "description": "The osmolar gap is the difference between measured serum osmolality (from the lab's freezing-point depression assay) and calculated osmolality (estimated from sodium, glucose, and BUN). A gap greater than roughly 10 mOsm/kg suggests the presence of unmeasured osmotically active substances, classically toxic alcohols like methanol, ethylene glycol, isopropyl alcohol, and ethanol itself, but also mannitol, severe hyperlipidemia, or paraproteinemia. It is used as a rapid screening tool in suspected toxic alcohol ingestion when specific levels are not immediately available, since a normal osmolar gap makes significant toxic alcohol ingestion less likely (though it does not fully exclude it, especially later in the course after the alcohol has been metabolized into its more toxic, less osmotically active acid metabolites). A falsely low or normal gap late in ingestion is a well-recognized pitfall — the anion gap metabolic acidosis from the toxic metabolites may already be present even as the osmolar gap normalizes. This tool is for education and workflow support only — not medical advice. Always verify results with a clinician and use clinical judgment alongside your institution's protocols.",
  "formula": "Calculated osm = 2×Na + glucose/18 + BUN/2.8; Osmolar gap = measured osm − calculated osm.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=osmolar-gap",
  "inputs": [
    {
      "id": "measuredOsm",
      "label": "Measured serum osmolality",
      "kind": "number",
      "hint": null,
      "default": 320,
      "unit": "mOsm/kg",
      "min": 250,
      "max": 450
    },
    {
      "id": "na",
      "label": "Sodium",
      "kind": "number",
      "hint": null,
      "default": 140,
      "unit": "mEq/L",
      "min": 110,
      "max": 170
    },
    {
      "id": "glucose",
      "label": "Glucose",
      "kind": "number",
      "hint": null,
      "default": 100,
      "unit": "mg/dL",
      "min": 40,
      "max": 800
    },
    {
      "id": "bun",
      "label": "BUN",
      "kind": "number",
      "hint": null,
      "default": 15,
      "unit": "mg/dL",
      "min": 2,
      "max": 150
    },
    {
      "id": "ethanol",
      "label": "Ethanol level (if known, else 0)",
      "kind": "number",
      "hint": null,
      "default": 0,
      "unit": "mg/dL",
      "min": 0,
      "max": 500
    }
  ],
  "outputs": [
    {
      "id": "gap",
      "label": "Osmolar gap",
      "format": "decimal",
      "hint": null,
      "primary": true
    },
    {
      "id": "calcOsm",
      "label": "Calculated osmolality",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "interpretation",
      "label": "Interpretation",
      "format": "raw",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Measured serum osmolality: 320 mOsm/kg",
      "Sodium: 140 mEq/L",
      "Glucose: 100 mg/dL",
      "BUN: 15 mg/dL",
      "Ethanol level (if known, else 0): 0 mg/dL"
    ],
    "outputs": [
      "Osmolar gap: 29.1",
      "Calculated osmolality: 290.9",
      "Interpretation: Elevated — consider toxic alcohol or other unmeasured osmoles"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter measured serum osmolality (mOsm/kg).",
      "Enter sodium (mEq/L).",
      "Enter glucose (mg/dL).",
      "Enter bun (mg/dL).",
      "Enter ethanol level (if known, else 0) (mg/dL).",
      "Read your osmolar gap on the right — it updates as you type.",
      "Hit Share to keep the scenario or send it to someone."
    ]
  },
  "scenarios": [
    {
      "name": "Conservative",
      "description": "Lower-end numbers — what if things land soft?",
      "values": {
        "measuredOsm": 250,
        "na": 110,
        "glucose": 60,
        "bun": 9,
        "ethanol": 0
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "measuredOsm": 320,
        "na": 140,
        "glucose": 100,
        "bun": 15,
        "ethanol": 0
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "measuredOsm": 450,
        "na": 170,
        "glucose": 160,
        "bun": 24,
        "ethanol": 0
      }
    }
  ],
  "limitations": [
    "Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.",
    "Results do not account for medical conditions, medication, or pregnancy.",
    "Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it."
  ],
  "faq": [
    {
      "q": "What counts as an elevated osmolar gap?",
      "a": "Above roughly 10 mOsm/kg is generally considered elevated, though 'normal' has real variability between labs and individuals — some healthy people run gaps up to 10-15 at baseline. Trend and clinical context matter more than a single cutoff, especially with suspected toxic ingestion."
    },
    {
      "q": "Can a normal osmolar gap rule out toxic alcohol poisoning?",
      "a": "No, not reliably, especially late in the course. As methanol or ethylene glycol is metabolized into its acidic byproducts, the osmotically active parent alcohol decreases while the anion gap acidosis from the toxic metabolites increases — so a normal osmolar gap late in ingestion can coexist with severe, ongoing toxicity."
    },
    {
      "q": "Why include ethanol in the calculated osmolality?",
      "a": "Ethanol is itself an osmotically active substance, and a patient who has been drinking will have an elevated measured osmolality from ethanol alone. Including a measured ethanol level in the calculation (divided by 4.6 to convert mg/dL to mOsm/kg contribution) prevents ethanol from falsely inflating the gap and pointing you toward a different toxic alcohol."
    },
    {
      "q": "What else besides toxic alcohols raises the osmolar gap?",
      "a": "Severe hypertriglyceridemia, paraproteinemia (as in multiple myeloma), mannitol administration, and severe renal failure with accumulated unmeasured solutes can all raise the osmolar gap. Clinical context — ingestion history, lipid panel, and renal function — helps narrow the differential."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/abg-interpretation",
    "https://www.revenuelab.fyi/toolbox/delta-gap-ratio",
    "https://www.revenuelab.fyi/toolbox/fena-calculator"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Osmolar Gap Calculator (https://www.revenuelab.fyi/toolbox/osmolar-gap)"
}