WEDGE

FEUrea

Compute fractional excretion of urea (FEUN) from BUN, urine urea nitrogen, plasma creatinine, and urine creatinine.

WEDGE $0.005 USDC GET /api/calc/feurea

Markdown: /docs/feurea.md · Canonical: https://magentlab.com/docs/feurea

Not a medical device. Not a diagnosis, dose, or listing recommendation.

What it computes

Compute fractional excretion of urea (FEUN) from BUN, urine urea nitrogen, plasma creatinine, and urine creatinine.

When to use

When diuretics make FENa unhelpful and an agent must not invent FEUrea from a language model.

When not to use

• Not a diagnosis of prerenal azotemia or ATN. Carvounis bands are published FEUN cut-points (low ≤35%, high >35%), not a clinical order.
• FENa is a separate tool. This calculator does not compute FENa.
• Diuretics and CKD can change interpretation. Arithmetic only from the four laboratory values.

Formula

FEUrea_percent = 100 * (urine_urea_mg_dl * plasma_creatinine_mg_dl) / (bun_mg_dl * urine_creatinine_mg_dl)

Citation

Title

Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure

Authors

Carvounis CP, Nisar S, Guro-Razuman S

Source

Kidney Int. 2002;62(6):2223-2229

DOI

10.1046/j.1523-1755.2002.00683.x

Worked example

Try

Opens the live endpoint. Unpaid browser requests show the paywall; agents should send Accept: application/json.

/api/calc/feurea?bun_mg_dl=20&urine_urea_mg_dl=200&creatinine_mg_dl=2&urine_creatinine_mg_dl=100

Full URL: https://api.magentlab.com/api/calc/feurea?bun_mg_dl=20&urine_urea_mg_dl=200&creatinine_mg_dl=2&urine_creatinine_mg_dl=100

x402 V2 curl

Expect HTTP 402 and a PAYMENT-REQUIRED header until you retry with PAYMENT-SIGNATURE. Incomplete query params return HTTP 400 before any quote or charge. HTTP 503 after a signature: retry the same PAYMENT-SIGNATURE — that is not a new 402.

curl -i -H "Accept: application/json" "https://api.magentlab.com/api/calc/feurea?bun_mg_dl=20&urine_urea_mg_dl=200&creatinine_mg_dl=2&urine_creatinine_mg_dl=100"

Cursor mcp.json

Intended config for @magent/mcp. This is not npm @x402/fetch (a payment fetch wrapper).

{
  "mcpServers": {
    "magent": {
      "args": [
        "-y",
        "@magent/mcp"
      ],
      "command": "npx",
      "env": {
        "X402_PRIVATE_KEY": "0xYOUR_SPENDING_KEY"
      }
    }
  }
}

Parameters

Bulk (POST)

POST the same path with a JSON items array. Price is n times the GET unit. Invalid items return HTTP 400 before settlement.

POST /api/calc/feurea Max 25 unit_amount * n PREVIEW

• 1 to 25 items. Each item uses the same keys as the GET query parameters.
• The first invalid item fails the whole request (index in the 400 JSON). You are not charged.
• HTTP 200 is {count, path, items}. Each items[i] is the GET 200 body for that row.
• Catalog misses (empty matches, found=false) stay paid 200s inside that slot.
• x402 resource URL is this path plus n and body_sha256. Browsers that prefer text/html get the same paywall as GET; Pay retries POST with the same items.

Example request (fixture)

{
  "items": [
    {
      "bun_mg_dl": "20",
      "creatinine_mg_dl": "2",
      "urine_creatinine_mg_dl": "100",
      "urine_urea_mg_dl": "200"
    },
    {
      "bun_mg_dl": "20",
      "creatinine_mg_dl": "2",
      "urine_creatinine_mg_dl": "100",
      "urine_urea_mg_dl": "200"
    }
  ]
}

Open bulk paywall →

Opens a two-item fixture on the API host. Unpaid browsers get the same MetaMask paywall as GET; Pay retries POST with those items.

Example 200 (fixture)

{
  "count": 2,
  "path": "/api/calc/feurea",
  "items": [
    {
      "units": "%",
      "disclaimer": "Not a medical device. Deterministic published-formula or published-code output for autonomous agents. A licensed clinician remains responsible for patient care and billing submissions.",
      "citation": {
        "authors": "Carvounis CP, Nisar S, Guro-Razuman S",
        "doi": "10.1046/j.1523-1755.2002.00683.x",
        "id": "carvounis-2002",
        "source": "Kidney Int. 2002;62(6):2223-2229",
        "title": "Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure"
      },
      "formula": "carvounis",
      "formula_expression": "FEUrea_percent = 100 * (urine_urea_mg_dl * plasma_creatinine_mg_dl) / (bun_mg_dl * urine_creatinine_mg_dl)",
      "creatinine_mg_dl": 2.0,
      "urine_creatinine_mg_dl": 100.0,
      "bun_mg_dl": 20.0,
      "carvounis_band": "low",
      "feurea_percent": 20.0,
      "urine_urea_mg_dl": 200.0
    },
    {
      "units": "%",
      "disclaimer": "Not a medical device. Deterministic published-formula or published-code output for autonomous agents. A licensed clinician remains responsible for patient care and billing submissions.",
      "citation": {
        "authors": "Carvounis CP, Nisar S, Guro-Razuman S",
        "doi": "10.1046/j.1523-1755.2002.00683.x",
        "id": "carvounis-2002",
        "source": "Kidney Int. 2002;62(6):2223-2229",
        "title": "Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure"
      },
      "formula": "carvounis",
      "formula_expression": "FEUrea_percent = 100 * (urine_urea_mg_dl * plasma_creatinine_mg_dl) / (bun_mg_dl * urine_creatinine_mg_dl)",
      "creatinine_mg_dl": 2.0,
      "urine_creatinine_mg_dl": 100.0,
      "bun_mg_dl": 20.0,
      "carvounis_band": "low",
      "feurea_percent": 20.0,
      "urine_urea_mg_dl": 200.0
    }
  ]
}

Response

Example JSON

Machine form of the same example. Frozen fixture. Not a live lookup.

{
  "units": "%",
  "disclaimer": "Not a medical device. Deterministic published-formula or published-code output for autonomous agents. A licensed clinician remains responsible for patient care and billing submissions.",
  "citation": {
    "authors": "Carvounis CP, Nisar S, Guro-Razuman S",
    "doi": "10.1046/j.1523-1755.2002.00683.x",
    "id": "carvounis-2002",
    "source": "Kidney Int. 2002;62(6):2223-2229",
    "title": "Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure"
  },
  "formula": "carvounis",
  "formula_expression": "FEUrea_percent = 100 * (urine_urea_mg_dl * plasma_creatinine_mg_dl) / (bun_mg_dl * urine_creatinine_mg_dl)",
  "creatinine_mg_dl": 2.0,
  "urine_creatinine_mg_dl": 100.0,
  "bun_mg_dl": 20.0,
  "carvounis_band": "low",
  "feurea_percent": 20.0,
  "urine_urea_mg_dl": 200.0
}

Errors

Related tools

• Cockcroft–Gault CrCl — Compute Cockcroft–Gault creatinine clearance in mL/min using total body weight.
• CKD-EPI 2021 eGFR — Compute race-free CKD-EPI 2021 creatinine eGFR in mL/min/1.73m2.
• FENa — Compute fractional excretion of sodium from plasma and urine sodium and creatinine using Espinel 1976.

Also searched as

• fractional excretion of urea
• FEUN
• FEUrea
• Carvounis FEUN
• urine urea nitrogen fraction
• fractional urea excretion

Payment

Required query parameters must be present and valid. Invalid params return HTTP 400 JSON before settlement. magent does not charge. Fix the params, then request a new 402 quote for that complete URL. Catalog misses (empty matches, found=false) are valid paid 200s. Unpaid valid requests return HTTP 402 with a base64 PAYMENT-REQUIRED header. HTTP 503 is not a new quote: retry the same PAYMENT-SIGNATURE. Full handshake and shared payment errors: Payments.

Guarantees

• Invalid query parameters return HTTP 400 before settlement. You are not charged.
• Settlement finishes before the tool executes. Uncertain or failed settlement fails closed (HTTP 503). Retry the same PAYMENT-SIGNATURE.
• HTTP 402 is unpaid only. Do not sign a second authorization because of a 503.
• Execution is timeout-bounded and concurrency-capped.
• Calculator inputs are not persisted. Request logs store path without query.