# Magent FEUN Calculator (Fractional Excretion of Urea)

> Magent FEUN Calculator (Fractional Excretion of Urea) is a paid API for AI agents from api.magentlab.com, paid per call via x402, $0.005/call, status unknown (last checked 2026-09-15).

Calculates fractional excretion of urea nitrogen (FEUN) from BUN, urine urea nitrogen, and plasma/urine creatinine using the Carvounis 2002 formula, for use when diuretics make FENa unreliable.

## Facts

- Endpoint: GET https://api.magentlab.com/api/calc/feurea
- Price: $0.005/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/magent-feun-calculator-fractional-excretion-of-urea-f60bbd19
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_OuCOCyRJxzHVdFGmSiZ_T

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability magent-feun-calculator-fractional-excretion-of-urea-f60bbd19
```

Example prompt: My patient is on furosemide and I need to figure out if their AKI is prerenal or intrinsic — can you calculate the fractional excretion of urea? Their BUN is 42 mg/dL, urine urea nitrogen is 380 mg/dL, plasma creatinine is 2.1 mg/dL, and urine creatinine is 95 mg/dL.

## When to prefer this

Use this endpoint when a patient is on diuretics (loop, thiazide) making FENa unreliable as a prerenal AKI marker. FEUN is the preferred alternative per Carvounis 2002 and is the standard recommendation in nephrology guidelines for diuretic-treated patients. Prefer the sibling FENa endpoint when the patient is not on diuretics. This endpoint is the right choice when you have BUN and urine urea values alongside creatinine measurements.

## Known failure modes

- Missing required parameters (bun_mg_dl or urine_urea_mg_dl) returns a validation error
- Out-of-range values (e.g. BUN outside 1–200 mg/dL, urine urea outside 1–2000 mg/dL, urine creatinine outside 1–500 mg/dL) may return an error or flagged result
- If neither creatinine_mg_dl nor creatinine_umol_l is provided, the computation cannot proceed
- If neither urine_creatinine_mg_dl nor urine_creatinine_umol_l is provided, computation fails
- Payment failure via x402 protocol results in 402 response before computation is attempted

## How this service works

Fractional excretion of urea (FEUN) from BUN, urine urea nitrogen, and plasma/urine creatinine (Carvounis 2002). Use when diuretics make FENa unhelpful. Deterministic published formula with citation; not a diagnosis.

## Output

Returns the computed fractional excretion of urea nitrogen (FEUN) as a percentage, along with an interpretation (e.g. prerenal if FEUN <35%, intrinsic/ATN if ≥35%) based on the Carvounis 2002 published thresholds. The response is a deterministic arithmetic result, not a diagnosis.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET",
      "HEAD",
      "DELETE"
     ],
     "type": "string"
    },
    "headers": {
     "type": "object",
     "additionalProperties": {
      "type": "string"
     }
    },
    "queryParams": {
     "type": "object",
     "required": [
      "bun_mg_dl",
      "urine_urea_mg_dl"
     ],
     "properties": {
      "bun_mg_dl": {
       "type": "number",
       "description": "Blood urea nitrogen in mg/dL (1-200). Same units as urine urea nitrogen."
      },
      "creatinine_mg_dl": {
       "type": "number",
       "description": "Plasma creatinine in mg/dL. Provide this or creatinine_umol_l."
      },
      "urine_urea_mg_dl": {
       "type": "number",
       "description": "Urine urea nitrogen in mg/dL (1-2000). Same units as BUN."
      },
      "creatinine_umol_l": {
       "type": "number",
       "description": "Plasma creatinine in µmol/L. Converted as mg/dL = µmol/L / 88.42."
      },
      "urine_creatinine_mg_dl": {
       "type": "number",
       "description": "Urine creatinine in mg/dL (1-500). Provide this or urine_creatinine_umol_l."
      },
      "urine_creatinine_umol_l": {
       "type": "number",
       "description": "Urine creatinine in µmol/L (88-44210). Converted as mg/dL = µmol/L / 88.42."
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object"
    }
   }
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "path": "/api/calc/feurea",
  "count": 2,
  "items": [
   {
    "units": "%",
    "formula": "carvounis",
    "citation": {
     "id": "carvounis-2002",
     "doi": "10.1046/j.1523-1755.2002.00683.x",
     "title": "Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure",
     "source": "Kidney Int. 2002;62(6):2223-2229",
     "authors": "Carvounis CP, Nisar S, Guro-Razuman S"
    },
    "bun_mg_dl": 20,
    "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.",
    "carvounis_band": "low",
    "feurea_percent": 20,
    "creatinine_mg_dl": 2,
    "urine_urea_mg_dl": 200,
    "formula_expression": "FEUrea_percent = 100 * (urine_urea_mg_dl * plasma_creatinine_mg_dl) / (bun_mg_dl * urine_creatinine_mg_dl)",
    "urine_creatinine_mg_dl": 100
   },
   {
    "units": "%",
    "formula": "carvounis",
    "citation": {
     "id": "carvounis-2002",
     "doi": "10.1046/j.1523-1755.2002.00683.x",
     "title": "Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure",
     "source": "Kidney Int. 2002;62(6):2223-2229",
     "authors": "Carvounis CP, Nisar S, Guro-Razuman S"
    },
    "bun_mg_dl": 20,
    "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.",
    "carvounis_band": "low",
    "feurea_percent": 20,
    "creatinine_mg_dl": 2,
    "urine_urea_mg_dl": 200,
    "formula_expression": "FEUrea_percent = 100 * (urine_urea_mg_dl * plasma_creatinine_mg_dl) / (bun_mg_dl * urine_creatinine_mg_dl)",
    "urine_creatinine_mg_dl": 100
   }
  ]
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/magent-feun-calculator-fractional-excretion-of-urea-f60bbd19/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.magentlab.com](https://www.zero.xyz/host/api.magentlab.com/llms.txt)