# Rule of 7s for Lyme meningitis

Rule of 7s

Status: LIVE
Price: $0.005 USDC
`GET /api/calc/rule_7s`
HTML: https://magentlab.com/docs/rule-7s
Markdown: https://magentlab.com/docs/rule-7s.md

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

## What it computes

Apply the Cohn 2012 Rule of 7s and return whether all three published low-risk criteria hold (rule7s_low_risk).

## When to use

When an agent already has headache duration in days, CSF mononuclear percent, and whether cranial-nerve palsy is present, as in Cohn 2012, and needs the published three-item Rule of 7s low-risk result. magent does not examine nerves or count CSF cells.

## When not to use

- Not a medical device.
- Not a Lyme diagnosis. Low-risk on the Rule of 7s is not a diagnosis of Lyme meningitis or aseptic meningitis.
- magent does not examine nerves or count CSF cells. headache_days, csf_mononuclear_percent, and cranial_nerve_palsy are caller-assigned.
- This tool is the named three-item Rule of 7s as stated in Cohn 2012. It does not compute Avery 2006 logistic probabilities.
- Not a treatment or discharge order.

## Formula

```
low-risk iff headache_days < 7 and csf_mononuclear_percent < 70 and cranial_nerve_palsy is false
```

## Citation

[Validation of a clinical prediction rule to distinguish Lyme meningitis from aseptic meningitis](https://doi.org/10.1542/peds.2011-1215)
Cohn KA, Thompson AD, Shah SS, Hines EM, Lyons TW, Welsh EJ, Nigrovic LE
Pediatrics. 2012;129(1):e46-e53
DOI: [10.1542/peds.2011-1215](https://doi.org/10.1542/peds.2011-1215)

## Worked example

### All three low-risk criteria hold

Given: `cranial_nerve_palsy=false, csf_mononuclear_percent=40, headache_days=3`

1. Headache duration 3 days is < 7
2. CSF mononuclear percent 40 is < 70
3. Cranial-nerve palsy is absent
4. Rule of 7s is low-risk


### Headache duration of 7 days is not low-risk

Given: `cranial_nerve_palsy=false, csf_mononuclear_percent=40, headache_days=7`

1. Headache duration 7 days is not < 7
2. CSF mononuclear percent and cranial-nerve palsy still meet the other two criteria
3. Rule of 7s is not low-risk


## Also searched as

- Rule of 7s
- Rule of 7's
- Cohn Rule of 7s
- Lyme meningitis rule
- Lyme vs aseptic meningitis
- Cohn 2012
- Rule of sevens

## Parameters

- `headache_days` (integer, required): Duration of headache in days (integer 0-90). Low-risk criterion holds when strictly less than 7. Seven days is not low-risk. magent does not take a history.
- `csf_mononuclear_percent` (number, required): CSF mononuclear cell percent (0-100). Low-risk criterion holds when strictly less than 70. Seventy percent is not low-risk. magent does not count CSF cells.
- `cranial_nerve_palsy` (boolean, required): Seventh or other cranial-nerve palsy. true or false as assigned by the caller. Low-risk criterion holds when false (palsy absent). magent does not examine nerves.

## 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.

Method: `POST /api/calc/rule_7s`
Max items: 25
Price: unit_amount + extra_item_amount * (n - 1) (unit 5000 atomic, extra 2000 atomic)
Status: 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:

```json
{
  "items": [
    {
      "cranial_nerve_palsy": "false",
      "csf_mononuclear_percent": "40",
      "headache_days": "3"
    },
    {
      "cranial_nerve_palsy": "false",
      "csf_mononuclear_percent": "40",
      "headache_days": "3"
    }
  ]
}
```

Human paywall (two-item fixture): https://api.magentlab.com/paywall/bulk/rule-7s

Example 200:

```json
{
  "count": 2,
  "path": "/api/calc/rule_7s",
  "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": "Cohn KA, Thompson AD, Shah SS, Hines EM, Lyons TW, Welsh EJ, Nigrovic LE",
        "doi": "10.1542/peds.2011-1215",
        "id": "cohn-2012",
        "pmid": "22184646",
        "source": "Pediatrics. 2012;129(1):e46-e53",
        "title": "Validation of a clinical prediction rule to distinguish Lyme meningitis from aseptic meningitis"
      },
      "formula": "rule_7s",
      "formula_expression": "low-risk iff headache_days < 7 and csf_mononuclear_percent < 70 and cranial_nerve_palsy is false",
      "criteria": [
        {
          "value": 3,
          "factor": "headache_days_lt_7",
          "met": true
        },
        {
          "value": 40.0,
          "factor": "csf_mononuclear_percent_lt_70",
          "met": true
        },
        {
          "factor": "no_cranial_nerve_palsy",
          "met": true
        }
      ],
      "rule7s_low_risk": true
    },
    {
      "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": "Cohn KA, Thompson AD, Shah SS, Hines EM, Lyons TW, Welsh EJ, Nigrovic LE",
        "doi": "10.1542/peds.2011-1215",
        "id": "cohn-2012",
        "pmid": "22184646",
        "source": "Pediatrics. 2012;129(1):e46-e53",
        "title": "Validation of a clinical prediction rule to distinguish Lyme meningitis from aseptic meningitis"
      },
      "formula": "rule_7s",
      "formula_expression": "low-risk iff headache_days < 7 and csf_mononuclear_percent < 70 and cranial_nerve_palsy is false",
      "criteria": [
        {
          "value": 3,
          "factor": "headache_days_lt_7",
          "met": true
        },
        {
          "value": 40.0,
          "factor": "csf_mononuclear_percent_lt_70",
          "met": true
        },
        {
          "factor": "no_cranial_nerve_palsy",
          "met": true
        }
      ],
      "rule7s_low_risk": true
    }
  ]
}
```

## Response fields

- `formula` (string): rule_7s
- `formula_expression` (string): Cohn 2012 three-item Rule of 7s low-risk conjunction
- `rule7s_low_risk` (boolean): true only when all three Cohn 2012 low-risk criteria hold. Not a Lyme diagnosis and not a treatment or discharge order.
- `criteria` (array): Per-factor rows with factor and met; met is true when that Cohn 2012 low-risk criterion holds
- `citation` (object): Cohn 2012 Pediatrics 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": "Cohn KA, Thompson AD, Shah SS, Hines EM, Lyons TW, Welsh EJ, Nigrovic LE",
    "doi": "10.1542/peds.2011-1215",
    "id": "cohn-2012",
    "pmid": "22184646",
    "source": "Pediatrics. 2012;129(1):e46-e53",
    "title": "Validation of a clinical prediction rule to distinguish Lyme meningitis from aseptic meningitis"
  },
  "formula": "rule_7s",
  "formula_expression": "low-risk iff headache_days < 7 and csf_mononuclear_percent < 70 and cranial_nerve_palsy is false",
  "criteria": [
    {
      "value": 3,
      "factor": "headache_days_lt_7",
      "met": true
    },
    {
      "value": 40.0,
      "factor": "csf_mononuclear_percent_lt_70",
      "met": true
    },
    {
      "factor": "no_cranial_nerve_palsy",
      "met": true
    }
  ],
  "rule7s_low_risk": true
}
```

## Errors

- HTTP 400 `invalid_headache_days`: headache_days must be an integer from 0 to 90 (Cohn 2012 Rule of 7s). Returned before settlement; you are not charged.
- HTTP 400 `invalid_csf_mononuclear`: csf_mononuclear_percent must be a number from 0 to 100 (Cohn 2012 Rule of 7s). Returned before settlement; you are not charged.
- HTTP 400 `invalid_flag`: boolean clinical flags must be true or false 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.

Shared HTTP 402 and 503: https://magentlab.com/docs/payments

## Related tools

- [Bacterial Meningitis Score](https://magentlab.com/docs/bacterial-meningitis) — Sum the Nigrovic 2002 Bacterial Meningitis Score weights for children with CSF pleocytosis and return very_low_risk (0) or not_low_risk (≥1).

## 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.

- 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 accepts[0] (exact, EIP-3009) with validBefore = now + maxTimeoutSeconds (600 seconds), or accepts[1] (batch-settlement deposit/voucher) on product GET. Ping and POST {items} stay exact. Magent retries transient facilitator 429/5xx before answering.
- Retry the same URL with PAYMENT-SIGNATURE (base64 JSON of accepted and payload). Magent attaches canonical extensions.bazaar on CDP verify/settle; echoing 402 bazaar is optional.
- Success is HTTP 200 JSON plus PAYMENT-RESPONSE. exact settles before the tool. batch-settlement verifies (and deposit-settles) before the tool and commits the charge after HTTP 2xx. HTTP 503: retry the same PAYMENT-SIGNATURE. Mint a new exact 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.
- offered `batch-settlement` on eip155:8453: Payment-channel vouchers claimed later in batches. Exact stays accepts[0]. Same-path POST {items} is still exact. extra.assetTransferMethod=eip3009 extra.paymentFlow=authorization.

- 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.

Same-path POST {items} is still scheme exact: one EIP-3009 authorization for GET unit plus $0.002 per extra item. That is not x402 batch-settlement (payment channels / vouchers).

Shared HTTP 402 and 503 table: https://magentlab.com/docs/payments

Stdio MCP (one process per host): https://magentlab.com/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.