{
  "id": "fena",
  "version": "d043ef74209b",
  "status": "published",
  "name": "FENa Calculator",
  "question": "What is the FENa?",
  "summary": "Calculates the fractional excretion of sodium (FENa) from serum and urine sodium and creatinine, in US or SI units, and shows how the Espinel cut-offs read it in acute kidney injury.",
  "category": "health",
  "subcategory": "clinical",
  "url": "https://www.acalculator.org/health/fena-calculator",
  "markdown": "https://www.acalculator.org/health/fena-calculator.md",
  "kind": "function",
  "method": "FENa (%) = (urine sodium × serum creatinine) ÷ (serum sodium × urine creatinine) × 100.",
  "assumptions": [
    "Serum and urine samples taken at about the same time, both creatinine values in the same unit.",
    "The FENa is rounded to 2 decimals and read from the rounded value: under 1% prerenal, 1% to 3% indeterminate, over 3% intrinsic kidney injury (Espinel 1976).",
    "The cut-offs were set for acute kidney injury with low urine output. Diuretics, chronic kidney disease, contrast dye, and early obstruction can make the FENa misleading."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "units": {
        "title": "Creatinine units",
        "description": "US units (mg/dL) or SI units (µmol/L) for both creatinine values.",
        "type": "string",
        "enum": [
          "us",
          "si"
        ]
      },
      "sNa": {
        "title": "Serum sodium (mmol/L)",
        "description": "Sodium in the blood, in mmol/L (the same number as mEq/L).",
        "type": "number",
        "minimum": 100,
        "maximum": 180
      },
      "uNa": {
        "title": "Urine sodium (mmol/L)",
        "description": "Sodium in a spot urine sample, in mmol/L.",
        "type": "number",
        "minimum": 1,
        "maximum": 400
      },
      "sCr": {
        "title": "Serum creatinine (mg/dL)",
        "description": "Creatinine in the blood, in mg/dL.",
        "type": "number",
        "minimum": 0.1,
        "maximum": 20
      },
      "uCr": {
        "title": "Urine creatinine (mg/dL)",
        "description": "Creatinine in the same urine sample, in mg/dL.",
        "type": "number",
        "minimum": 1,
        "maximum": 1000
      },
      "sCrSi": {
        "title": "Serum creatinine (µmol/L)",
        "description": "Creatinine in the blood, in µmol/L.",
        "type": "number",
        "minimum": 8.84,
        "maximum": 1768
      },
      "uCrSi": {
        "title": "Urine creatinine (µmol/L)",
        "description": "Creatinine in the same urine sample, in µmol/L.",
        "type": "number",
        "minimum": 88.4,
        "maximum": 88400
      }
    }
  },
  "outputs": {
    "fena": {
      "label": "FENa",
      "description": "Fractional excretion of sodium, the percent of filtered sodium that reaches the urine, to 2 decimals.",
      "format": "percent"
    },
    "reading": {
      "label": "What the cut-offs say",
      "description": "How the Espinel cut-offs read this FENa in a person with acute kidney injury and low urine output.",
      "format": "text"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "units": "us",
      "sNa": 140,
      "uNa": 20,
      "sCr": 2,
      "uCr": 100,
      "sCrSi": 177,
      "uCrSi": 8840
    },
    "outputs": {
      "fena": 0.29,
      "reading": "Under 1%: fits a prerenal cause"
    },
    "text": "The FENa is 0.29%. Under 1%: fits a prerenal cause."
  },
  "examples": [
    {
      "given": {
        "units": "us",
        "sNa": 140,
        "uNa": 20,
        "sCr": 2,
        "uCr": 100
      },
      "expect": {
        "fena": 0.29,
        "reading": "Under 1%: fits a prerenal cause"
      },
      "source": "Espinel 1976: FENa = (UNa × PCr) ÷ (PNa × UCr) × 100 = (20 × 2) ÷ (140 × 100) × 100 = 0.2857%, shown as 0.29% (hand calculation in content.mdx)"
    },
    {
      "given": {
        "units": "us",
        "sNa": 135,
        "uNa": 60,
        "sCr": 4,
        "uCr": 50
      },
      "expect": {
        "fena": 3.56,
        "reading": "Over 3%: fits intrinsic kidney injury (such as ATN)"
      },
      "source": "Espinel 1976 (over 3% in acute tubular necrosis): (60 × 4) ÷ (135 × 50) × 100 = 3.5556%, shown as 3.56% (hand calculation in content.mdx)"
    },
    {
      "given": {
        "units": "si",
        "sNa": 138,
        "uNa": 40,
        "sCrSi": 221,
        "uCrSi": 6630
      },
      "expect": {
        "fena": 0.97,
        "reading": "Under 1%: fits a prerenal cause"
      },
      "source": "(40 × 221) ÷ (138 × 6,630) × 100 = 0.9662%, shown as 0.97%; the unit cancels (hand calculation in content.mdx)"
    },
    {
      "given": {
        "units": "us",
        "sNa": 140,
        "uNa": 28,
        "sCr": 1,
        "uCr": 20
      },
      "expect": {
        "fena": 1,
        "reading": "1% to 3%: indeterminate"
      },
      "source": "(28 × 1) ÷ (140 × 20) × 100 = 1.00%, on the 1% cut-off, which is indeterminate (hand calculation)"
    }
  ],
  "sources": [
    "Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581. https://doi.org/10.1001/jama.1976.03270060029022",
    "Miller TR, Anderson RJ, Linas SL, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47-50. https://doi.org/10.7326/0003-4819-89-1-47",
    "KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int Suppl. 2013;3(1):1-150. https://kdigo.org/guidelines/ckd-evaluation-and-management/ (1 mg/dL creatinine = 88.4 µmol/L)"
  ],
  "related": [],
  "changelog": [
    {
      "date": "2026-09-29",
      "note": "Published after this site's editorial review against the cited sources (no clinician review); the page notice says so. The upper cut-off is now over 3% (Espinel 1976), not over 2%."
    }
  ]
}