Fractional excretion of sodium calculator

Enter paired plasma and urine sodium concentrations together with paired plasma and urine creatinine concentrations, and the calculator applies the standard FENa equation. The computation is fully offline, deterministic, and transparent: the result includes the formula and normalized inputs used. It is intended as a calculation aid for qualified interpretation, not as a diagnosis or a substitute for clinical assessment.

Enter a matched set of four measurements

FENa requires four positive measurements: plasma sodium, urine sodium, plasma creatinine, and urine creatinine. Use values collected closely enough in time to represent the same clinical state, because combining unrelated samples can produce a mathematically valid percentage that does not describe a meaningful physiologic moment. Sodium units must match each other, so plasma and urine sodium may both be entered in mmol/L or both in mEq/L. Creatinine units must also match each other, such as mg/dL for both plasma and urine creatinine. The sodium unit does not need to match the creatinine unit because each ratio cancels within its own analyte. Enter bare numeric concentrations without unit symbols, commas, inequality signs, or explanatory text. Every value must be finite and strictly greater than zero. A missing value, zero, negative value, nonnumeric string, NaN, or infinity produces an invalid-input error instead of a speculative answer. Check decimal placement and confirm that urine and plasma labels have not been reversed before using the result elsewhere.

Understand the deterministic calculation

The calculator uses FENa (%) = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100. Creatinine acts as a filtration reference, allowing sodium excretion to be expressed as a fraction of the filtered sodium load rather than as a urine concentration alone. For example, dilute or concentrated urine can change a raw urine sodium measurement, while the paired creatinine relationship supplies needed context within the equation. The implementation performs only constant-time arithmetic after validating the four fields. It does not contact a laboratory system, infer missing measurements, choose substitute values, use a model, or retain data. To make automated comparisons stable, the returned percentage and echoed inputs are normalized to twelve significant digits. The response also states the percentage unit and exact formula string, making it practical to audit a calculation or reproduce it independently. Because the operation is deterministic, identical valid inputs always produce the same JSON result in the browser and through the API.

Interpret the percentage with clinical context

FENa is a derived laboratory index, not a standalone diagnosis. Thresholds are often discussed in evaluation of acute kidney injury, but their meaning depends on the patient, timing, treatment, kidney function, and the clinical question. Diuretics can alter urinary sodium handling, while sepsis, contrast exposure, glomerular disease, obstruction, chronic kidney disease, and other settings may weaken simple threshold-based interpretations. A value should therefore be reviewed alongside urine output, volume assessment, trends in creatinine, medication exposure, urinalysis, and other relevant findings. This capability deliberately returns the computed percentage without assigning a disease category or recommending treatment. It also does not validate whether a measurement method, specimen timing, or chosen unit is clinically appropriate; it verifies only that the four supplied values are positive finite numbers. Qualified clinicians should apply local protocols and current guidance. If a result is surprising, first verify transcription, specimen pairing, and unit consistency, then recalculate rather than changing an input merely to obtain an expected category.