# @who-growth/core

WHO Child Growth Standards calculator — z-scores, percentiles, and nutritional classification.

Zero dependencies. Works in Node.js, browsers, and edge runtimes.

## Features

- **2 chart sets**: WHO standard, Down syndrome (Zemel 2015)
- **6 growth indicators**: weight-for-age, length/height-for-age, BMI-for-age, head circumference-for-age, weight-for-length, weight-for-height
- **Full age range**: 0–19 years (WHO), 0–18 years (Down syndrome)
- **Prematurity correction**: gestational age adjustment up to 2 years
- **Nutritional classification**: SISVAN/WHO cutpoints
- **i18n**: pt-BR, en, es
- **LMS data**: 26 official tables embedded, lazy-loaded

## Chart Sets

| Chart Set | Status | Age Range | Indicators |
|-----------|--------|-----------|------------|
| WHO Standard | ✅ Complete | 0–19 years | All 6 |
| Down Syndrome (Zemel 2015) | ✅ Complete | 0–20 years | WFA (0–18y), LFA (0–18y), BFA (0–18y), HCFA (0–20y) |

## Install

```bash
npm install @who-growth/core
```

## Usage

```typescript
import { calculateZScore, calculateAll } from '@who-growth/core';

// Single indicator (WHO standard)
const result = await calculateZScore({
  indicator: 'weight-for-age',
  sex: 'male',
  ageInDays: 365,
  measurement: 9.5,
});
// → { indicator: 'weight-for-age', zScore: -0.11, percentile: 45.6 }

// Full assessment (WHO standard)
const assessment = await calculateAll({
  sex: 'female',
  dateOfBirth: new Date('2024-06-15'),
  dateOfMeasurement: new Date('2025-06-15'),
  weight: 9.2,
  lengthHeight: 74.5,
  headCircumference: 45,
  gestationalAgeWeeks: 34, // optional — triggers prematurity correction
});
// → { age, results, classifications }

// Down syndrome charts
const dsResult = await calculateAll({
  sex: 'male',
  dateOfBirth: new Date('2024-01-01'),
  dateOfMeasurement: new Date('2025-01-01'),
  weight: 8.5,
  lengthHeight: 70,
  chartSet: 'down-syndrome',
});
```

## API

### `calculateZScore(input): Promise<ZScoreResult | null>`

Calculate z-score and percentile for a single indicator.

**Options:**
- `chartSet`: `'who-standard'` (default) | `'down-syndrome'`

### `calculateAll(input): Promise<AssessmentResult>`

Calculate all applicable indicators for a patient, including age calculation, prematurity correction, and nutritional classification.

### `computeZScore(measurement, lms): number`

Low-level z-score calculation from measurement and LMS parameters. Implements the WHO standard formula with L=0 handling and |Z|>3 extrapolation.

### `classify(result, ageInDays): Classification`

Classify a z-score result using SISVAN/WHO cutpoints.

### `ageInDays(dateOfBirth, dateOfMeasurement): number`

Calculate age in days between two dates.

### `correctedAgeInDays(chronologicalDays, gestationalAgeWeeks?): number`

Apply prematurity correction (up to 2 years of age).

## Formulas

- **Standard**: `Z = ((measurement/M)^L - 1) / (L × S)`
- **When L=0**: `Z = ln(measurement/M) / S`
- **|Z| > 3**: WHO linear extrapolation method
- **Percentile**: Normal CDF (Abramowitz & Stegun approximation)

## Data Sources

- **WHO**: [WHO Anthro](https://github.com/WorldHealthOrganization/anthro) (0–5y) + [WHO AnthroPlus](https://github.com/WorldHealthOrganization/anthroplus) (5–19y)
- **Down Syndrome**: Zemel BS, et al. *Growth Charts for Children With Down Syndrome in the United States.* Pediatrics. 2015;136(5):e1204-e1211.

## License

MIT