R/pct_for_age_generic.R

In ronkeizer/PKmisc: Functions for clinical pharmacokinetics

#' Percentile height or weight for age for children
#'
#' This is the underlying function, the exposed functions are pct_weight_for_age() and pct_height_for_age()
#' Based on tables from WHO: http://www.who.int/childgrowth/standards/height_for_age/en/
#'
#' @param age age in years
#' @param sex either `male` or `female`
#' @param value height in kg. Optional, if specified, will calculate closest percentile and return in list as `percentile`
#' @param variable weight or height?
#' @param ... parameters passed to `read_who_table()`
pct_for_age_generic <- function(age = NULL, value = NULL, sex = NULL, variable="weight", ...) {
  if(is.null(age) || is.null(sex)) {
    stop("Age and sex are required!")
  }
  if(variable == "height") {
    if(age > 19) {
      message("Sorry, height data currently only available for age <= 19 years.")
      return(NULL)
    }
  }
  if(variable == "weight") {
    if(age > 10) {
      message("Sorry, currently only available for age <= 10 years.")
      return(NULL)
    }
  }
  type <- "wfa"
  if(variable == "height") {
    type = "lhfa"
    if(age >= 5.1) {
      type = "hfa" # naming inconsistyency from WHO
    }
  }
  if(variable == "bmi") {
    type <- "bfa"
    if(age >= 5.1) {
      type <- "bmi"
    }
  }
  dat <- read_who_table(sex=sex, age=age, type=type, download=FALSE)
  tmp <- dat[order(abs(age - dat[,1])),][1,-(1:4)]
  pct <- as.list(tmp)
  if(!is.null(value)) {
    p <- c()
    for(i in seq(names(pct))) {
      p <- c(p, as.num(gsub("P", "", names(pct)[i])))
    }
    p[1] <- p[1]/10 # 0.1
    p[length(p)] <- p[length(p)]/10 # 99.9
    p_txt <- paste0("pct_", p)
    if(value > max(tmp)) {
      message(paste0("Specified ", variable," > 99.9th percentile!"))
      pct <- list(percentile = 99.9)
    }
    if(value < min(tmp)) {
      message(paste0("Specified ", variable, " < 0.1th percentile!"))
      pct <- list(percentile = 0.1)
    }
    if(is.null(pct$percentile)) {
      data <- data.frame(cbind(
        x = c(as.num(tmp[value <= as.num(tmp)][1]), tail(as.num(tmp[value > as.num(tmp)]),1)),
        y = c(p[value <= as.num(tmp)][1], tail(p[value > as.num(tmp)],1))
      ))
      # linearly scale between two values
      fit <- lm(y~x, data)
      par <- coef(fit)
      pct <- list(percentile = round(as.num(par[1] + par[2]*value),1))
    }
  }
  return(pct)
}