Electrolyte Panel Calculator
Calculate anion gap, BUN/creatinine ratio, and serum osmolality from a basic metabolic panel (BMP) for electrolyte assessment.
About this calculator
This calculator derives three commonly used ratios from a basic metabolic panel. The anion gap (AG = Na+ - (Cl- + HCO3-), normal roughly 8-12 mEq/L) flags whether an unmeasured acid may be present, prompting a search for causes like diabetic ketoacidosis, lactic acidosis, uremia, or certain toxic ingestions when elevated. The BUN/creatinine ratio (normal roughly 10-20) helps distinguish prerenal causes of kidney injury -- where reduced blood flow to the kidneys causes BUN to rise disproportionately to creatinine, pushing the ratio above 20 -- from intrinsic renal causes, where both tend to rise together and the ratio stays closer to normal.
Calculated osmolality (2 x Na+ + glucose/18 + BUN/2.8, normal roughly 275-295 mOsm/kg) is an ADA-endorsed estimate used especially in hyperglycemic crises, and its difference from a lab-measured osmolality -- the osmolal gap -- is a screening tool: a gap wider than roughly 10 mOsm/kg suggests an unmeasured osmotically active substance is present in the blood, classic culprits being ethanol, methanol, ethylene glycol, or mannitol. None of these three numbers is a diagnosis by itself; each narrows a differential that still needs the rest of the clinical picture to resolve.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
Results
Anion Gap
14 mEq/L (normal 8-12)
BUN/Creatinine Ratio
15 (normal 10-20)
How to Use This Calculator
- Enter serum sodium (mEq/L), potassium (mEq/L), chloride (mEq/L), and bicarbonate (mEq/L).
- Enter BUN and creatinine (mg/dL), and glucose (mg/dL).
- Review the calculated anion gap: AG = Na - (Cl + HCO3); normal = 8-12 mEq/L.
- Review the BUN/Creatinine ratio (normal ~10-20) and calculated osmolality (normal ~275-295 mOsm/kg).
- Enter a lab-measured osmolality to compute the osmolal gap — a gap above ~10 mOsm/kg suggests an unmeasured toxin (e.g. ethanol, methanol, ethylene glycol).
- Correlate electrolyte abnormalities with patient medications, hydration status, and clinical picture.
How the result changes with Sodium (Na+)
| Sodium (Na+) | Anion Gap | BUN/Creatinine Ratio |
|---|---|---|
| 100 | -26 mEq/L (normal 8-12) | 15 (normal 10-20) |
| 105 | -21 mEq/L (normal 8-12) | 15 (normal 10-20) |
| 180 | 54 mEq/L (normal 8-12) | 15 (normal 10-20) |
What each input means
- Sodium (Na+)
- Serum sodium level (normal 136-145 mEq/L).
- Potassium (K+)
- Serum potassium level (normal 3.5-5.0 mEq/L).
- Chloride (Cl-)
- Serum chloride level (normal 98-106 mEq/L).
- Bicarbonate (HCO3-)
- Serum bicarbonate / CO2 level (normal 22-29 mEq/L).
- BUN
- Blood urea nitrogen level (normal 7-20 mg/dL).
- Creatinine
- Serum creatinine level (normal 0.7-1.3 mg/dL).
- Glucose
- Blood glucose level (fasting normal 70-100 mg/dL).
- Measured Osmolality
- Lab-measured serum osmolality (freezing-point depression), used to compute the osmolal gap against the calculated value below. Normal range ~275-295 mOsm/kg.
What each result means
- Osmolal Gap
- A gap greater than roughly 10 mOsm/kg suggests an unmeasured osmotically active substance (e.g. ethanol, methanol, ethylene glycol).
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSodium (Na+) = 140, Potassium (K+) = 4.2, Chloride (Cl-) = 102, Bicarbonate (HCO3-) = 24 = 7 input(s) provided
- Calculate Anion GapAnion Gap14 = 14
- Calculate BUN/Creatinine RatioBUN/Creatinine Ratio15 = 15
- Calculate Calculated OsmolalityCalculated Osmolality290.9 = 290.9
- Calculate Osmolal GapOsmolal Gap-0.9 = -0.9
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does sodium move the anion gap more than potassium, BUN, or glucose here?
Potassium, BUN, and glucose don't appear in the anion gap formula at all -- AG is calculated only from sodium, chloride, and bicarbonate (AG = Na+ - Cl- - HCO3-). Sodium also has the largest normal reference value of the three terms that DO appear in the formula, so an equivalent percentage change in sodium moves the calculated gap by a larger absolute amount than an equivalent percentage change in chloride or bicarbonate.
What does a BUN/creatinine ratio above 20 suggest?
A ratio above roughly 20 (with the normal range around 10-20) often points toward a prerenal cause of reduced kidney function -- something reducing blood flow to otherwise healthy kidneys, such as dehydration, heart failure, or blood loss -- because BUN reabsorption increases disproportionately to creatinine when blood flow to the kidneys is low. A ratio that stays closer to normal despite rising creatinine points more toward an intrinsic kidney problem instead.
What does the osmolal gap actually screen for?
It's the difference between a lab-measured serum osmolality and the calculated estimate this calculator produces from sodium, glucose, and BUN. A gap wider than roughly 10 mOsm/kg suggests something osmotically active is in the blood that the calculated formula doesn't account for -- classic causes include ethanol, methanol, ethylene glycol, isopropyl alcohol, and mannitol -- which is why it's used as a rapid toxic-alcohol screening tool in the emergency department.
Is the calculated osmolality the same as a lab-measured osmolality?
No -- calculated osmolality is only an estimate derived from three routine BMP values (sodium, glucose, and BUN), while a lab-measured osmolality is determined directly by freezing-point depression and captures every osmotically active particle in the blood, measured or not. The two are meant to be compared, not treated as interchangeable -- their difference is precisely what the osmolal gap is designed to reveal.
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