# FENa Calculator (Fractional Excretion of Sodium) > FENa Calculator (Fractional Excretion of Sodium) 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). Computes fractional excretion of sodium (FENa%) from plasma sodium, urine sodium, plasma creatinine, and urine creatinine using the Espinel 1976 formula for oliguric acute renal failure stratification. ## Facts - Endpoint: GET https://api.magentlab.com/api/calc/fena - 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/fena-calculator-fractional-excretion-of-sodium-60af7850 - Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_fhslXtNBxYAhNpgvD2rin 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 fena-calculator-fractional-excretion-of-sodium-60af7850 ``` Example prompt: Calculate FENa for my patient with oliguria: plasma sodium is 138 mmol/L, urine sodium is 18 mmol/L, serum creatinine is 2.4 mg/dL, and urine creatinine is 120 mg/dL — what does the Espinel 1976 stratification say? ## When to prefer this Use this endpoint when you need a deterministic, citation-backed FENa calculation (Espinel 1976) for oliguric ARF stratification and want automatic unit conversion between mg/dL and µmol/L for creatinine inputs. Prefer over manual calculation or LLM-estimated values when reproducibility and published-formula fidelity matter. ## Known failure modes - Missing required parameter sodium_mmol_l or urine_sodium_mmol_l returns 400 error - No creatinine value provided (neither mg/dL nor µmol/L for plasma or urine) returns validation error - Values outside physiologic ranges (e.g. sodium < 110 or > 160 mmol/L) may be rejected - Both creatinine unit variants provided simultaneously may cause ambiguity ## How this service works Fractional excretion of sodium (FENa) from plasma and urine sodium and creatinine. Espinel 1976 oliguric ARF strata. Deterministic published formula with citation; not a diagnosis. ## Output Returns the computed FENa percentage along with the Espinel 1976 oliguric ARF stratum (e.g., prerenal vs. intrinsic renal failure classification), based on the deterministic published formula. Not a clinical 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": [ "sodium_mmol_l", "urine_sodium_mmol_l" ], "properties": { "sodium_mmol_l": { "type": "number", "description": "Plasma or serum sodium in mmol/L (110-160)." }, "creatinine_mg_dl": { "type": "number", "description": "Plasma creatinine in mg/dL (0.1-20). Provide this or creatinine_umol_l." }, "creatinine_umol_l": { "type": "number", "description": "Plasma creatinine in µmol/L (9-1768). Converted as mg/dL = µmol/L / 88.42." }, "urine_sodium_mmol_l": { "type": "number", "description": "Urine sodium in mmol/L (1-300)." }, "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/fena", "count": 2, "items": [ { "formula": "fena", "citation": { "id": "espinel-1976", "doi": "10.1001/jama.1976.03270060029022", "title": "The FENa test. Use in the differential diagnosis of acute renal failure", "source": "JAMA. 1976;236(6):579-581", "authors": "Espinel CH" }, "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.", "fena_percent": 0.29, "sodium_mmol_l": 140, "espinel_stratum": "prerenal", "creatinine_mg_dl": 2, "formula_expression": "FENa_percent = 100 * (urine_sodium_mmol_l * plasma_creatinine_mg_dl) / (plasma_sodium_mmol_l * urine_creatinine_mg_dl)", "urine_sodium_mmol_l": 20, "urine_creatinine_mg_dl": 100 }, { "formula": "fena", "citation": { "id": "espinel-1976", "doi": "10.1001/jama.1976.03270060029022", "title": "The FENa test. Use in the differential diagnosis of acute renal failure", "source": "JAMA. 1976;236(6):579-581", "authors": "Espinel CH" }, "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.", "fena_percent": 0.29, "sodium_mmol_l": 140, "espinel_stratum": "prerenal", "creatinine_mg_dl": 2, "formula_expression": "FENa_percent = 100 * (urine_sodium_mmol_l * plasma_creatinine_mg_dl) / (plasma_sodium_mmol_l * urine_creatinine_mg_dl)", "urine_sodium_mmol_l": 20, "urine_creatinine_mg_dl": 100 } ] } } ``` ## More - Live health (JSON, refreshed every minute): https://www.zero.xyz/c/fena-calculator-fractional-excretion-of-sodium-60af7850/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)