Stool osmotic gap (Eherer–Fordtran)

Stool osmotic gap

Compute the Eherer–Fordtran fecal osmotic gap from stool sodium and potassium, using measured stool osmolality when supplied or 290 mOsm/kg when omitted.

LIVE $0.005 USDC GET /api/calc/stool_osmotic_gap

Markdown: /docs/stool-osmotic-gap.md · Canonical: https://magentlab.com/docs/stool-osmotic-gap

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

What it computes

Compute the Eherer–Fordtran fecal osmotic gap from stool sodium and potassium, using measured stool osmolality when supplied or 290 mOsm/kg when omitted.

When to use

When an agent needs the published fecal osmotic-gap arithmetic and must not invent an osmotic-versus-secretory diagnosis.

When not to use

Not a diagnosis of osmotic versus secretory diarrhea. gap_gt_50 flags whether the gap exceeds the paper’s ~50 mOsm/kg separator; it is not a clinical classification.
When measured stool osmolality is omitted, 290 mOsm/kg is assumed. A negative gap is returned when 2×(Na+K) exceeds the osmolality used (for example from laboratory error) and is not rejected.

Formula

osmotic_gap = stool_osm_mosm_kg - 2 * (stool_sodium_mmol_l + stool_potassium_mmol_l)

Citation

Title: Fecal osmotic gap and pH in experimental diarrhea of various causes
Authors: Eherer AJ, Fordtran JS
Source: Gastroenterology. 1992;103(2):545-551
DOI: 10.1016/0016-5085(92)90845-P

Worked example

Stool Na 30, K 75, assumed osm 290

Given: stool_potassium_mmol_l=75stool_sodium_mmol_l=30

2 × (30 + 75) = 210
assumed stool osm = 290 mOsm/kg
osmotic_gap = 290 − 210 = 80.0

Also searched as

fecal osmotic gap
stool osmotic gap
stool osmolar gap
fecal osm gap
Eherer Fordtran gap
stool sodium potassium gap

Try

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

/api/calc/stool_osmotic_gap?stool_sodium_mmol_l=30&stool_potassium_mmol_l=75

Full URL: https://api.magentlab.com/api/calc/stool_osmotic_gap?stool_sodium_mmol_l=30&stool_potassium_mmol_l=75

Call

Expect HTTP 402 until you retry with PAYMENT-SIGNATURE. Incomplete query params return HTTP 400 before any quote or charge.

curl -i -H "Accept: application/json" "https://api.magentlab.com/api/calc/stool_osmotic_gap?stool_sodium_mmol_l=30&stool_potassium_mmol_l=75"

Parameters

Name	Type	Required	Description
`stool_sodium_mmol_l`	number	required	Stool sodium in mmol/L (1-150).
`stool_potassium_mmol_l`	number	required	Stool potassium in mmol/L (1-150).
`stool_osm_mosm_kg`	number	optional	Measured stool osmolality in mOsm/kg (200-600). When omitted, 290 mOsm/kg is assumed.

Bulk (POST)

POST the same path with a JSON items array. The first item is the GET unit; each additional item is $0.002. Invalid items return HTTP 400 before settlement.

POST /api/calc/stool_osmotic_gap Max 25 unit_amount + extra_item_amount * (n - 1) extra $0.002 PREVIEW

1 to 25 items. Each item uses the same keys as the GET query parameters.
POST of 1 item costs the same as GET. Lists longer than 25: chunk into multiple POSTs. Extra items still execute; do not pad dummy rows.
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": [
    {
      "stool_potassium_mmol_l": "75",
      "stool_sodium_mmol_l": "30"
    },
    {
      "stool_potassium_mmol_l": "75",
      "stool_sodium_mmol_l": "30"
    }
  ]
}

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/stool_osmotic_gap",
  "items": [
    {
      "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": "Eherer AJ, Fordtran JS",
        "doi": "10.1016/0016-5085(92)90845-P",
        "id": "eherer-fordtran-1992",
        "source": "Gastroenterology. 1992;103(2):545-551",
        "title": "Fecal osmotic gap and pH in experimental diarrhea of various causes"
      },
      "formula": "eherer_fordtran",
      "formula_expression": "osmotic_gap = stool_osm_mosm_kg - 2 * (stool_sodium_mmol_l + stool_potassium_mmol_l)",
      "assumed_osmolality": true,
      "gap_gt_50": true,
      "osmotic_gap": 80.0,
      "stool_osm_mosm_kg": 290.0,
      "stool_potassium_mmol_l": 75.0,
      "stool_sodium_mmol_l": 30.0
    },
    {
      "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": "Eherer AJ, Fordtran JS",
        "doi": "10.1016/0016-5085(92)90845-P",
        "id": "eherer-fordtran-1992",
        "source": "Gastroenterology. 1992;103(2):545-551",
        "title": "Fecal osmotic gap and pH in experimental diarrhea of various causes"
      },
      "formula": "eherer_fordtran",
      "formula_expression": "osmotic_gap = stool_osm_mosm_kg - 2 * (stool_sodium_mmol_l + stool_potassium_mmol_l)",
      "assumed_osmolality": true,
      "gap_gt_50": true,
      "osmotic_gap": 80.0,
      "stool_osm_mosm_kg": 290.0,
      "stool_potassium_mmol_l": 75.0,
      "stool_sodium_mmol_l": 30.0
    }
  ]
}

Response

Field	Type	Description
`formula`	string	eherer_fordtran
`formula_expression`	string	Exact expression used
`stool_sodium_mmol_l`	number	Input stool sodium in mmol/L
`stool_potassium_mmol_l`	number	Input stool potassium in mmol/L
`stool_osm_mosm_kg`	number	Stool osmolality used in mOsm/kg (measured or assumed 290)
`assumed_osmolality`	boolean	true when stool_osm_mosm_kg was omitted and 290 mOsm/kg was used
`osmotic_gap`	number	Fecal osmotic gap in mOsm/kg, 1 decimal. May be negative.
`gap_gt_50`	boolean	true when osmotic_gap is greater than 50, the Eherer–Fordtran separator. Not a diagnosis.
`citation`	object	Eherer and Fordtran 1992 citation
`disclaimer`	string	Not-a-device notice

Example JSON

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

{
  "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": "Eherer AJ, Fordtran JS",
    "doi": "10.1016/0016-5085(92)90845-P",
    "id": "eherer-fordtran-1992",
    "source": "Gastroenterology. 1992;103(2):545-551",
    "title": "Fecal osmotic gap and pH in experimental diarrhea of various causes"
  },
  "formula": "eherer_fordtran",
  "formula_expression": "osmotic_gap = stool_osm_mosm_kg - 2 * (stool_sodium_mmol_l + stool_potassium_mmol_l)",
  "assumed_osmolality": true,
  "gap_gt_50": true,
  "osmotic_gap": 80.0,
  "stool_osm_mosm_kg": 290.0,
  "stool_potassium_mmol_l": 75.0,
  "stool_sodium_mmol_l": 30.0
}

Client errors (HTTP 400)

Invalid params return 400 without charge. Shared HTTP 402 and 503 meanings: Payments.

HTTP	Code	When
400	`invalid_stool_sodium`	stool_sodium_mmol_l must be a number from 1 to 150. Returned before settlement; you are not charged.
400	`invalid_stool_potassium`	stool_potassium_mmol_l must be a number from 1 to 150. Returned before settlement; you are not charged.
400	`invalid_stool_osmolality`	stool_osm_mosm_kg must be a number from 200 to 600 when provided. Returned before settlement; you are not charged.
400	`invalid_items`	POST body must be a JSON object with only an items array Returned before settlement; you are not charged.
400	`invalid_item_count`	items must be an array of 1 to 25 objects Returned before settlement; you are not charged.
400	`invalid_item`	each items entry must be a JSON object Returned before settlement; you are not charged.

Related tools

Serum osmolality — Compute Worthley calculated serum osmolality from sodium, glucose, and BUN, plus an optional osmolar gap when a measured osm is supplied.

Payment

Unpaid requests return HTTP 402 even if query parameters or POST items are missing or invalid, so CDP and agents can index the path. After a PAYMENT-SIGNATURE, 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. Stdio MCP: /docs/mcp.

Guarantees

HTTP 402 is unpaid, including a missing query. After PAYMENT-SIGNATURE, invalid parameters return HTTP 400 before settlement. You are not charged. HTTP 402 never includes the tool JSON.
exact: settlement finishes before the tool (paymentFlow upfront). batch-settlement: verify/deposit before the tool; chargedCumulativeAmount commits after HTTP 2xx. 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 stored encrypted for allowlisted operators (Magent Console). They are not in public logs, SSH lookups, catalog, or rollups. Request logs store path without query.