{
  "slug": "fena-calculator",
  "title": "Fractional Excretion of Sodium (FeNa) Calculator",
  "heading": "FeNa Calculator",
  "category": "fitness-health",
  "url": "https://www.revenuelab.fyi/toolbox/fena-calculator",
  "summary": "Distinguish prerenal from intrinsic acute kidney injury.",
  "description": "Fractional excretion of sodium compares how much filtered sodium the kidney excretes versus reabsorbs, and it helps distinguish prerenal azotemia (kidneys appropriately conserving sodium and water in response to poor perfusion) from intrinsic acute kidney injury like acute tubular necrosis (tubules damaged and unable to reabsorb sodium normally), in a patient with acute kidney injury and oliguria. A FeNa below 1% suggests prerenal physiology — the kidney is working correctly but underperfused — while a FeNa above 2% suggests intrinsic renal damage. Values between 1-2% are indeterminate. FeNa is unreliable after diuretic administration (diuretics force sodium excretion regardless of underlying cause), in chronic kidney disease, and in some intrinsic causes like contrast nephropathy or early sepsis-associated AKI that can present with a low FeNa despite intrinsic damage — in diuretic-exposed patients, fractional excretion of urea (FeUrea) is used instead since urea handling is less affected by diuretics. 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": "FeNa (%) = [(Urine Na × Plasma Cr) ÷ (Plasma Na × Urine Cr)] × 100.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=fena-calculator",
  "inputs": [
    {
      "id": "urineNa",
      "label": "Urine sodium",
      "kind": "number",
      "hint": null,
      "default": 15,
      "unit": "mEq/L",
      "min": 0,
      "max": 300
    },
    {
      "id": "plasmaNa",
      "label": "Plasma sodium",
      "kind": "number",
      "hint": null,
      "default": 138,
      "unit": "mEq/L",
      "min": 100,
      "max": 170
    },
    {
      "id": "urineCr",
      "label": "Urine creatinine",
      "kind": "number",
      "hint": null,
      "default": 60,
      "unit": "mg/dL",
      "min": 1,
      "max": 500
    },
    {
      "id": "plasmaCr",
      "label": "Plasma creatinine",
      "kind": "number",
      "hint": null,
      "default": 2.4,
      "unit": "mg/dL",
      "min": 0.2,
      "max": 20
    },
    {
      "id": "onDiuretic",
      "label": "Recent diuretic use",
      "kind": "select",
      "hint": null,
      "default": "no",
      "options": [
        {
          "value": "yes",
          "label": "Yes"
        },
        {
          "value": "no",
          "label": "No"
        }
      ]
    }
  ],
  "outputs": [
    {
      "id": "fena",
      "label": "FeNa",
      "format": "decimal",
      "hint": null,
      "primary": true
    },
    {
      "id": "interpretation",
      "label": "Interpretation",
      "format": "raw",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Urine sodium: 15 mEq/L",
      "Plasma sodium: 138 mEq/L",
      "Urine creatinine: 60 mg/dL",
      "Plasma creatinine: 2.4 mg/dL",
      "Recent diuretic use: No"
    ],
    "outputs": [
      "FeNa: 0.43",
      "Interpretation: Prerenal azotemia pattern"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter urine sodium (mEq/L).",
      "Enter plasma sodium (mEq/L).",
      "Enter urine creatinine (mg/dL).",
      "Enter plasma creatinine (mg/dL).",
      "Enter recent diuretic use.",
      "Read your fena 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": {
        "urineNa": 9,
        "plasmaNa": 100,
        "urineCr": 36,
        "plasmaCr": 1.44,
        "onDiuretic": "no"
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "urineNa": 15,
        "plasmaNa": 138,
        "urineCr": 60,
        "plasmaCr": 2.4,
        "onDiuretic": "no"
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "urineNa": 24,
        "plasmaNa": 170,
        "urineCr": 96,
        "plasmaCr": 3.84,
        "onDiuretic": "no"
      }
    }
  ],
  "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": "Why doesn't FeNa work after diuretics?",
      "a": "Diuretics act directly on renal sodium reabsorption, forcing sodium into the urine regardless of whether the underlying process is prerenal or intrinsic. This artificially raises FeNa in a patient who may actually have simple volume depletion, making the test misleading. Fractional excretion of urea is preferred in that setting because diuretics affect urea handling much less."
    },
    {
      "q": "What FeNa values point to prerenal versus intrinsic AKI?",
      "a": "Below 1% suggests prerenal azotemia — the kidney's concentrating mechanism is intact and appropriately conserving sodium in response to reduced perfusion. Above 2% suggests intrinsic damage, most often acute tubular necrosis, where damaged tubules can't reabsorb sodium normally. The 1-2% zone is a gray area requiring clinical correlation."
    },
    {
      "q": "Are there exceptions where a low FeNa doesn't mean prerenal?",
      "a": "Yes — early sepsis-associated AKI, contrast-induced nephropathy, and acute glomerulonephritis can present with a deceptively low FeNa despite being intrinsic processes, because these conditions can initially preserve tubular sodium handling. Clinical context, urinalysis, and urine microscopy for casts or eosinophils add important complementary information."
    },
    {
      "q": "Do I need a 24-hour urine collection for this?",
      "a": "No — FeNa uses a single spot urine and simultaneous plasma sample, which is what makes it practical at the bedside compared to timed collections. Just be sure the urine and blood samples are drawn close together in time for an accurate ratio."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/creatinine-clearance-cockcroft-gault",
    "https://www.revenuelab.fyi/toolbox/osmolar-gap",
    "https://www.revenuelab.fyi/toolbox/maintenance-fluid-rate"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Fractional Excretion of Sodium (FeNa) Calculator (https://www.revenuelab.fyi/toolbox/fena-calculator)"
}