# Stool osmotic gap

Status: LIVE
Price: $0.005 USDC
`GET /api/calc/stool_osmotic_gap`
HTML: https://magentlab.com/docs/stool-osmotic-gap
Markdown: https://magentlab.com/docs/stool-osmotic-gap.md

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

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

## Worked example

### Stool Na 30, K 75, assumed osm 290

Given: `stool_potassium_mmol_l=75, stool_sodium_mmol_l=30`

1. 2 × (30 + 75) = 210
2. assumed stool osm = 290 mOsm/kg
3. osmotic_gap = 290 − 210 = 80.0


## Parameters

- `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. Price is n times the GET unit. Invalid items return HTTP 400 before settlement.

Method: `POST /api/calc/stool_osmotic_gap`
Max items: 25
Price: unit_amount * n (unit 5000 atomic)
Status: 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:

```json
{
  "items": [
    {
      "stool_potassium_mmol_l": "75",
      "stool_sodium_mmol_l": "30"
    },
    {
      "stool_potassium_mmol_l": "75",
      "stool_sodium_mmol_l": "30"
    }
  ]
}
```

Human paywall (two-item fixture): https://api.magentlab.com/paywall/bulk/stool-osmotic-gap

Example 200:

```json
{
  "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 fields

- `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

Frozen fixture. Not a live lookup.

```json
{
  "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
}
```

## Errors

- HTTP 400 `invalid_stool_sodium`: stool_sodium_mmol_l must be a number from 1 to 150. Returned before settlement; you are not charged.
- HTTP 400 `invalid_stool_potassium`: stool_potassium_mmol_l must be a number from 1 to 150. Returned before settlement; you are not charged.
- HTTP 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.
- HTTP 400 `invalid_items`: POST body must be a JSON object with only an items array Returned before settlement; you are not charged.
- HTTP 400 `invalid_item_count`: items must be an array of 1 to 25 objects Returned before settlement; you are not charged.
- HTTP 400 `invalid_item`: each items entry must be a JSON object Returned before settlement; you are not charged.
- HTTP 402 `payment_required`: Missing or invalid PAYMENT-SIGNATURE; decode the PAYMENT-REQUIRED header
- HTTP 409 `payment_in_progress`: The same authorization nonce is already being settled; retry shortly
- HTTP 429 `rate_limited`: Too many requests from this client
- HTTP 503 `settlement_uncertain`: Settlement is unconfirmed; retry with the same PAYMENT-SIGNATURE
- HTTP 503 `settle_failed`: Settlement failed; retry with the same PAYMENT-SIGNATURE shortly
- HTTP 503 `facilitator_unavailable`: Payment facilitator unavailable
- HTTP 503 `facilitator_misconfigured`: Payment facilitator is not configured
- HTTP 503 `facilitator_unauthorized`: Payment facilitator rejected credentials
- HTTP 503 `ledger_unavailable`: Payment ledger unavailable
- HTTP 503 `payment_unavailable`: Payment processing unavailable

## Related tools

- [Serum osmolality](https://magentlab.com/docs/serum-osmolality) — Compute Worthley calculated serum osmolality from sodium, glucose, and BUN, plus an optional osmolar gap when a measured osm is supplied.

## Also searched as

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

## 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.

- GET the tool with Accept: application/json (do not send Accept: text/html unless you want the human paywall).
- HTTP 402 means you have not paid. Decode the base64 PAYMENT-REQUIRED header. HTTP 503 is not a new quote.
- Sign EIP-3009 (scheme exact, assetTransferMethod eip3009) with validBefore = now + accepts[0].maxTimeoutSeconds (600 seconds). That window starts when you sign and must cover wallet confirmation plus CDP verify and settle. Magent retries transient facilitator 429/5xx before answering. Do not send upto or batch-settlement payloads; magent does not offer those schemes.
- Retry the same URL with PAYMENT-SIGNATURE (base64 JSON echoing accepted, payload, and extensions when present).
- Success is HTTP 200 JSON plus PAYMENT-RESPONSE. Settlement happens before the tool runs. HTTP 503 settlement_uncertain or settle_failed: retry the same PAYMENT-SIGNATURE. Mint a new nonce only after a new HTTP 402.

- offered `exact` on eip155:8453: Fixed-price USDC on Base. The buyer authorizes the advertised amount (EIP-3009). magent settles before the tool runs (paymentFlow upfront). extra.assetTransferMethod=eip3009 extra.paymentFlow=upfront.

- not offered `upto`: Not offered. Usage-based: buyer authorizes a ceiling, seller settles the actual amount after work. Reserved for a future metered tool. Not for current calculators or code search.
- not offered `batch-settlement`: Not offered. Payment-channel vouchers claimed later in batches. Not the same as POST {items} bulk (that is still exact).

Same-path POST {items} is still scheme exact: one EIP-3009 authorization for n times the GET unit. That is not x402 batch-settlement (payment channels / vouchers).

## Guarantees

- Invalid query parameters return HTTP 400 before settlement. You are not charged.
- Settlement finishes before the tool executes (paymentFlow upfront). 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.