Urine Anion Gap Calculator - UAG, NH4+ Estimate, RTA Band

The urine anion gap calculator turns urine sodium, urine potassium, and urine chloride from a urine chemistry panel into the UAG in mEq per liter, an optional 24-hour NH4+ equivalent, and a positive, near zero, or negative band that frames the next clinical step in a hyperchloremic metabolic acidosis workup.

• • Hyperchloremic metabolic acidosis workup: Decide whether a normal serum anion gap acidosis fits a renal tubular acidosis pattern (positive UAG) or a gastrointestinal bicarbonate loss pattern (negative UAG).
• • Kidney acid excretion review: Spot check whether the kidney is appropriately excreting ammonium in chronic kidney disease or a urinary diversion.
• • 24-hour ammonium estimate: When a 24-hour urine volume is added, scale a per liter UAG into an indicative NH4+ excretion rate in mmol per day.
• • Bedside teaching and case review: Walk students or junior clinicians through the same calculation done on a real urine chemistry panel without losing the units.

The calculator is a screening tool for clinicians, trainees, and careful patients who already have a urine chemistry panel in hand.

Because the UAG is most useful when read with the serum value from the same draw, Anion Gap Calculator is the natural pair to compute the serum anion gap from the same basic metabolic panel.

The UAG is the difference between the major measured urine cations and the major measured urine anion. Add urine sodium and urine potassium, then subtract urine chloride, to get the UAG in mEq per liter. When a 24-hour urine volume is also entered, the same UAG is scaled into an indicative 24-hour NH4+ equivalent.

All four inputs come from the same urine collection so the units stay consistent. Sodium and potassium add to the cation side of the equation, chloride sits on the anion side, and the optional 24-hour volume is entered only when a real timed collection is available.

Bicarbonate is intentionally left out of the formula. The serum AG uses bicarbonate because serum is alkaline, but urine is acidic, so urine bicarbonate is negligible compared with chloride.

The NH4+ equivalent is a planning estimate, not a direct ammonium measurement. It only returns a non-zero value when the UAG is negative.

According to Merck Manual Professional Edition on metabolic acidosis, the urine anion gap is calculated as urine sodium plus urine potassium minus urine chloride in milliequivalents per liter, the published reference window is 0 to 10 mEq per liter with an extended range of -10 to 20 mEq per liter, and a positive UAG in a normal serum AG metabolic acidosis is the classic screening signature of impaired renal ammonium excretion.

Once the UAG points to renal or GI acid handling, Acid Base Calculator is the broader workup that adds urine pH and serum bicarbonate to the picture.

Four ideas cover why the published equation leaves bicarbonate out, why a negative UAG can be a reassuring finding, and why the UAG is only an indirect estimate of ammonium.

A common trap is to compare a UAG result to the same numeric thresholds as a serum AG. The serum AG uses 8 to 12 mEq per liter, while the UAG uses 0 to 10 mEq per liter because the urinary bicarbonate term is dropped.

In a chronic kidney disease review, the same urine chemistry panel used for the UAG should be paired with an estimated GFR, so GFR Calculator belongs in the same workup sheet.

The UAG is most useful when the urine chemistry panel, the serum electrolyte panel, and the 24-hour collection all come from the same workup window.

1. 1 Pull the urine chemistry panel: Collect urine sodium, urine potassium, and urine chloride in mEq per liter from the same urine chemistry panel, paired with a serum basic metabolic panel from the same draw.
2. 2 Enter the three urine electrolytes: Type each value into the matching field. Leave a value at 0 only if the lab returned an undetectable result.
3. 3 Decide on a 24-hour urine volume: If a 24-hour urine collection is available, enter the total volume in mL so the calculator can scale the UAG into an NH4+ equivalent. Leave at 0 for a spot urine read.
4. 4 Read the UAG and the band: The primary number is the UAG in mEq per liter. The band reads Positive above 10, Near zero between 0 and 10, and Negative below 0.
5. 5 Pair the band with the serum AG: A positive UAG plus a normal serum AG is the classic RTA pattern. A negative UAG plus a normal serum AG fits a gastrointestinal bicarbonate loss workup.
6. 6 Bring the result to a clinician: The band is a screening anchor, not a stand alone diagnosis. Bring the band, the underlying values, and the serum electrolyte panel to the clinician managing the case.

A urine chemistry panel with urine sodium 20, urine potassium 25, and urine chloride 55 mEq per liter paired with a normal serum AG gives a UAG of -10 mEq per liter and a Negative band, consistent with a gastrointestinal bicarbonate loss workup. The same case with urine chloride 15 returns a UAG of 40 mEq per liter and a Positive band that flags an RTA workup.

When the UAG band and the GFR together suggest impaired acid handling, Kidney Failure Risk Calculator is a useful second opinion on the trajectory of the kidney function.

The calculator is most useful when it turns a urine chemistry panel into a number that fits cleanly into a broader acid base workup.

• • Fast UAG with units that match the lab: Returns the UAG in mEq per liter using the same units as the source urine chemistry panel.
• • Positive, near zero, or negative band: Shows a clear band against the published 0 to 10 mEq per liter reference window.
• • Optional 24-hour NH4+ equivalent: Scales a per liter UAG into an indicative 24-hour ammonium excretion rate when a 24-hour urine volume is provided.
• • Built-in clinical interpretation: Names an RTA pattern for a positive UAG and a gastrointestinal bicarbonate loss pattern for a negative UAG.
• • Pairs with the serum AG: Returns the same numeric style as the serum AG calculator, so a serum AG and a UAG can be compared on the same workup sheet.

If the band is positive and the serum AG is normal, the next steps are urine pH, a serum potassium, and a review of the same urine chemistry panel.

If the band is negative and the serum AG is normal, the next steps are a stool history, a review of recent laxative or bowel surgery history, and a urine pH.

When the broader workup moves into a high serum anion gap metabolic acidosis, Diabetic Ketoacidosis Calculator keeps the same panel interpretation in one place rather than starting the math from scratch.

The UAG is a screening value, so it depends on the urine chemistry panel, the collection method, and the broader acid base context.

• • The calculator does not measure urinary ammonium directly. The 24-hour NH4+ equivalent is a planning estimate, so a laboratory urine ammonium assay is still the right test when the clinical decision depends on the number.
• • The result is not a stand alone diagnosis. A positive UAG can occur in a healthy subject on a very low salt day, and a negative UAG can occur in a subject with chronic diarrhea who is also on a loop diuretic. Always read the band with the serum AG, the urine pH, and the clinical context.
• • The calculator is a screening tool. It is not a substitute for a full acid base workup, an RTA workup, or a pediatric nephrology evaluation when the case is unusual.

The most common reason the UAG and the clinical picture disagree is a urine collection issue. If the band does not fit the case, ask the lab to confirm the units, the collection method, and the sample handling before changing the workup.

According to Merck Manual Professional Edition on renal tubular acidosis, renal tubular acidosis is a class of disorders that presents as a chronic metabolic acidosis with a normal anion gap, the workup is built around urine pH and urine electrolytes, and the urine anion gap is the indirect ammonium excretion estimate that separates an impaired renal ammonium response from a gastrointestinal bicarbonate loss pattern.

According to NEJM Batlle 1988 urinary anion gap study, a positive UAG in a patient with a normal serum AG metabolic acidosis is consistent with renal tubular acidosis, while a negative UAG is consistent with gastrointestinal bicarbonate loss.