# CKiD U25 eGFR

CKiD U25 eGFR

Status: LIVE
Price: $0.005 USDC
`GET /api/calc/ckid_u25`
HTML: https://magentlab.com/docs/ckid-u25
Markdown: https://magentlab.com/docs/ckid-u25.md

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

## What it computes

Compute Pierce 2021 CKiD U25 eGFR in mL/min/1.73m2 from age-dependent K with height and creatinine, cystatin C, or both.

## When to use

When an agent needs the CKiD under-25 eGFR for ages 1–25 and must not invent Schwartz 2009 bedside 0.413, adult CKD-EPI, or a CKD stage.

## When not to use

- Not a medical device. magent does not dose drugs.
- Validated for ages 1–25 years (CKiD under-25 equations). Not an adult CKD-EPI equation.
- This is the Pierce 2021 age-dependent K model (creatinine, cystatin C, or the average of both). It is not the Schwartz 2009 bedside 0.413 formula and not the constant-K CKiD models.
- Does not assign CKD stages.

## Formula

```
egfr_cr = K_cr * (height_m / creatinine_mg_dl); egfr_cys = K_cys * (1 / cystatin_c_mg_l); combined = (egfr_cr + egfr_cys) / 2 when both markers; K_cr and K_cys are Pierce 2021 Table 2 age-dependent (not constant-K models); height in meters; ages 1-25
```

## Citation

[Age- and sex-dependent clinical equations to estimate glomerular filtration rates in children and young adults with chronic kidney disease](https://doi.org/10.1016/j.kint.2020.10.047)
Pierce CB, Muñoz A, Ng DK, Warady BA, Furth SL, Schwartz GJ
Kidney Int. 2021;99(4):948-956
DOI: [10.1016/j.kint.2020.10.047](https://doi.org/10.1016/j.kint.2020.10.047)

## Worked example

### Male age 10, 140 cm, Scr 1.0 mg/dL

Given: `age_years=10, creatinine_mg_dl=1.0, height_cm=140, sex=male`

1. K_cr (male 1–<12) = 39.0 × 1.008^(10 − 12) = 39.0 / 1.016064 ≈ 38.3835
2. height_m = 140 / 100 = 1.40
3. eGFR = 38.3835 × (1.40 / 1.0) ≈ 53.7


### Female age 22, cystatin C 1.0 mg/L

Given: `age_years=22, cystatin_c_mg_l=1.0, sex=female`

1. K_cys (female 18–25) = 68.3
2. eGFR = 68.3 × (1 / 1.0) = 68.3


### Male age 10, 140 cm, Scr 1.0 mg/dL, cystatin C 1.2 mg/L

Given: `age_years=10, creatinine_mg_dl=1.0, cystatin_c_mg_l=1.2, height_cm=140, sex=male`

1. eGFR_cr ≈ 53.7 from the creatinine path
2. K_cys (male 1–<15) = 87.2 × 1.011^(10 − 15)
3. combined eGFR = (eGFR_cr + eGFR_cys) / 2


## Also searched as

- CKiD U25
- pediatric eGFR
- CKiD under 25
- Pierce 2021 eGFR
- age-dependent K eGFR
- cystatin C pediatric eGFR
- CKiD creatinine cystatin

## Parameters

- `age_years` (number, required): Age in years (1-25, fractional ages allowed). Pierce 2021 CKiD U25 age-dependent K is evaluated at that age. Not the adult 18-120 clamp.
- `sex` (string, required): Sex used by Pierce 2021 Table 2 K. female or male.
- `height_cm` (number, optional): Height in centimeters (50-250). Required with creatinine for the creatinine or combined path. Converted to meters in the equation. Provide height_cm or height_in. Not required for cystatin-only.
- `height_in` (number, optional): Height in inches (20-98). Converted as cm = in * 2.54.
- `creatinine_mg_dl` (number, optional): Serum creatinine in mg/dL (0.1-20). Required with height for the creatinine or combined path. Provide this or creatinine_umol_l.
- `creatinine_umol_l` (number, optional): Serum creatinine in µmol/L (9-1768). Converted as mg/dL = µmol/L / 88.42.
- `cystatin_c_mg_l` (number, optional): Serum cystatin C in mg/L (0.2-10). Required for the cystatin-only or combined path. Height is not required when this is the only marker.

## 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/ckid_u25`
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": [
    {
      "age_years": "10",
      "creatinine_mg_dl": "1.0",
      "height_cm": "140",
      "sex": "male"
    },
    {
      "age_years": "10",
      "creatinine_mg_dl": "1.0",
      "height_cm": "140",
      "sex": "male"
    }
  ]
}
```

Human paywall (two-item fixture): https://api.magentlab.com/paywall/bulk/ckid-u25

Example 200:

```json
{
  "count": 2,
  "path": "/api/calc/ckid_u25",
  "items": [
    {
      "units": "mL/min/1.73m2",
      "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": "Pierce CB, Muñoz A, Ng DK, Warady BA, Furth SL, Schwartz GJ",
        "doi": "10.1016/j.kint.2020.10.047",
        "id": "pierce-2021-ckid-u25",
        "pmid": "33301749",
        "source": "Kidney Int. 2021;99(4):948-956",
        "title": "Age- and sex-dependent clinical equations to estimate glomerular filtration rates in children and young adults with chronic kidney disease"
      },
      "formula": "ckid_u25_cr",
      "formula_expression": "egfr_cr = K_cr * (height_m / creatinine_mg_dl); egfr_cys = K_cys * (1 / cystatin_c_mg_l); combined = (egfr_cr + egfr_cys) / 2 when both markers; K_cr and K_cys are Pierce 2021 Table 2 age-dependent (not constant-K models); height in meters; ages 1-25",
      "age_years": 10.0,
      "creatinine_mg_dl": 1.0,
      "sex": "male",
      "height_cm": 140.0,
      "egfr_ml_min_1_73m2": 53.7,
      "k_cr": 38.3834
    },
    {
      "units": "mL/min/1.73m2",
      "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": "Pierce CB, Muñoz A, Ng DK, Warady BA, Furth SL, Schwartz GJ",
        "doi": "10.1016/j.kint.2020.10.047",
        "id": "pierce-2021-ckid-u25",
        "pmid": "33301749",
        "source": "Kidney Int. 2021;99(4):948-956",
        "title": "Age- and sex-dependent clinical equations to estimate glomerular filtration rates in children and young adults with chronic kidney disease"
      },
      "formula": "ckid_u25_cr",
      "formula_expression": "egfr_cr = K_cr * (height_m / creatinine_mg_dl); egfr_cys = K_cys * (1 / cystatin_c_mg_l); combined = (egfr_cr + egfr_cys) / 2 when both markers; K_cr and K_cys are Pierce 2021 Table 2 age-dependent (not constant-K models); height in meters; ages 1-25",
      "age_years": 10.0,
      "creatinine_mg_dl": 1.0,
      "sex": "male",
      "height_cm": 140.0,
      "egfr_ml_min_1_73m2": 53.7,
      "k_cr": 38.3834
    }
  ]
}
```

## Response fields

- `formula` (string): ckid_u25_cr, ckid_u25_cys, or ckid_u25_cr_cys
- `formula_expression` (string): Exact expression used
- `age_years` (number): Age in years used for K
- `sex` (string): female or male
- `height_cm` (number): Normalized height in cm. Omitted on the cystatin-only path.
- `creatinine_mg_dl` (number): Normalized creatinine in mg/dL. Omitted on the cystatin-only path.
- `cystatin_c_mg_l` (number): Cystatin C in mg/L. Omitted on the creatinine-only path.
- `k_cr` (number): Pierce 2021 Table 2 age-dependent K_cr, 4 decimals. Omitted when unused.
- `k_cys` (number): Pierce 2021 Table 2 age-dependent K_cys, 4 decimals. Omitted when unused.
- `egfr_ml_min_1_73m2` (number): CKiD U25 eGFR, 1 decimal place
- `units` (string): mL/min/1.73m2
- `citation` (object): Pierce 2021 Kidney International citation
- `disclaimer` (string): Not-a-device notice

## Example JSON

Frozen fixture. Not a live lookup.

```json
{
  "units": "mL/min/1.73m2",
  "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": "Pierce CB, Muñoz A, Ng DK, Warady BA, Furth SL, Schwartz GJ",
    "doi": "10.1016/j.kint.2020.10.047",
    "id": "pierce-2021-ckid-u25",
    "pmid": "33301749",
    "source": "Kidney Int. 2021;99(4):948-956",
    "title": "Age- and sex-dependent clinical equations to estimate glomerular filtration rates in children and young adults with chronic kidney disease"
  },
  "formula": "ckid_u25_cr",
  "formula_expression": "egfr_cr = K_cr * (height_m / creatinine_mg_dl); egfr_cys = K_cys * (1 / cystatin_c_mg_l); combined = (egfr_cr + egfr_cys) / 2 when both markers; K_cr and K_cys are Pierce 2021 Table 2 age-dependent (not constant-K models); height in meters; ages 1-25",
  "age_years": 10.0,
  "creatinine_mg_dl": 1.0,
  "sex": "male",
  "height_cm": 140.0,
  "egfr_ml_min_1_73m2": 53.7,
  "k_cr": 38.3834
}
```

## Errors

- HTTP 400 `invalid_ckid_u25`: CKiD U25 needs age 1-25, sex, and either height plus creatinine, cystatin C, or both, as used in Pierce 2021. Returned before settlement; you are not charged.
- HTTP 400 `invalid_sex`: sex must be female or male Returned before settlement; you are not charged.
- HTTP 400 `invalid_height`: height_cm (50-250) or height_in (20-98) is required Returned before settlement; you are not charged.
- HTTP 400 `invalid_creatinine`: creatinine_mg_dl (0.1-20) or creatinine_umol_l (9-1768) is required Returned before settlement; you are not charged.
- HTTP 400 `invalid_cystatin_c`: cystatin_c_mg_l must be a number from 0.2 to 10. 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

- [Schwartz 2009 eGFR](https://magentlab.com/docs/schwartz-gfr) — Compute the 2009 Schwartz bedside CKiD eGFR in mL/min/1.73m2 from height and serum creatinine.
- [CKD-EPI 2021 eGFR](https://magentlab.com/docs/egfr) — Compute race-free CKD-EPI 2021 creatinine eGFR in mL/min/1.73m2.
- [KFRE](https://magentlab.com/docs/kfre) — Compute 2-year and 5-year risk of kidney failure (dialysis or preemptive transplant) from the Tangri 2011 4-variable Kidney Failure Risk Equation (age, sex, eGFR, urine ACR).

## 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.