R/bsa.R

In LS31/clinicalr: Clinical calculations

#' Estimate the body surface area (BSA)
#'
#' Estimate the body surface area (BSA), according to Monsteller or Du Bois.
#'
#' @references [Simplified Calculation of Body-Surface Area. N Engl J Med 317, 1098–1098 (1987).](https://doi.org/10.1056\NEJM198710223171717) and [Du BOIS, D. CLINICAL CALORIMETRY. Arch Intern Med (Chic) XVII, 863 (1916).](https://doi.org/10.1001/archinte.1916.00080130010002).
#'
#' @section Caveats: None at this time.
#'
#' @param weight Weight (kg).
#' @param height Height (cm).
#' @param method Formula to estimate BSA. Options are: "Monsteller" (default),
#'   "Du Bois".
#' @return BSA (\eqn{m^2}), or `NA` if any parameters are `NA`.
#' @export
#' @seealso [units::set_units()], [units::drop_units()]
estimate_bsa <- function(weight, height, method = "Monsteller") {
  switch(
    method,
    "Monsteller" = estimate_bsa_monsteller(weight, height),
    "Du Bois" = estimate_bsa_du_bois(weight, height),
    stop("Illegal value for argument method in calculate_bsa().")
  )
}

#' @describeIn estimate_bsa Estimate the body surface area (BSA) according to Monsteller.
#' @noRd
estimate_bsa_monsteller <- function(weight, height) {
  if (anyNA(c(weight, height))) {
    return(NA)
  }
  assertthat::assert_that(assertthat::is.number(weight) |
                            is.na(weight))
  assertthat::assert_that(assertthat::is.number(height) |
                            is.na(height))

  ((weight * height / 3600) ^ (0.5)) %>%
    units::set_units("m-2", mode = "standard")
}

#' @describeIn estimate_bsa Estimate the body surface area (BSA) according to Du Bois
#' @noRd
estimate_bsa_du_bois <- function(weight, height) {
  if (anyNA(c(weight, height))) {
    return(NA)
  }
  assertthat::assert_that(assertthat::is.number(weight) |
                            is.na(weight))
  assertthat::assert_that(assertthat::is.number(height) |
                            is.na(height))

  (0.007184 * (weight ^ 0.425) * (height ^ 0.725)) %>%
    units::set_units("m-2", mode = "standard")
}