Transtubular Potassium Gradient Calculator

How it is calculated

The tool computes TTKG = (urine K × serum osmolality) ÷ (serum K × urine osmolality). This is the same as (urine K ÷ (urine osm ÷ serum osm)) ÷ serum K: the osmolality ratio estimates how much the urine potassium was concentrated by water reabsorption, and dividing by serum potassium compares it with the blood level. Potassium for urine and serum must be in the same unit (mmol/L or mEq/L, which are numerically equal for potassium), and the two osmolalities must also share a unit (mOsm/kg). Serum potassium and urine osmolality must be above zero. The result has no unit and is shown to two decimals. The tool gives only the number; it does not label it as high or low. Interpret it with the clinical picture and your laboratory's reference ranges, because it is a bedside estimate, not a diagnosis.

Common mistakes

• Swapping the osmolality values. The ratio is urine osmolality over serum osmolality inside the formula, so reversing them gives a very different TTKG.
• Mixing units between urine and serum potassium, for example mg/dL with mmol/L. Both must use the same unit.
• Using samples taken at different times. Urine and serum should be collected together, or the comparison loses meaning.
• Entering osmolarity from one sample and osmolality from the other. Use the same type of measurement for both.
• Reading the number alone as a diagnosis. The tool reports the calculation; deciding what it means needs the clinical context and a clinician.