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#' Calculate Low Blood Glucose Index (LBGI)
#'
#' @description
#' The function lbgi produces LBGI values in a tibble object.
#'
#' @usage
#' lbgi(data)
#'
#' @param data DataFrame object with column names "id", "time", and "gl",
#' or numeric vector of glucose values.
#'
#' @return If a data.frame object is passed, then a tibble object with
#' two columns: subject id and corresponding LBGI value is returned. If a vector of glucose
#' values is passed, then a tibble object with just the LBGI value is returned.
#' as.numeric() can be wrapped around the latter to output just a numeric value.
#'
#' @export
#'
#' @details
#' A tibble object with 1 row for each subject, a column for subject id and
#' a column for LBGI values is returned. NA glucose values are
#' omitted from the calculation of the LBGI.
#'
#' LBGI is calculated by \eqn{1/n * \sum (10 * fbg_i ^2)},
#' where \eqn{fbg_i = min(0, 1.509 * (log(G_i)^{1.084} - 5.381)},
#' G_i is the ith Glucose measurement for a subject, and
#' n is the total number of measurements for that subject.
#'
#' @references
#' Kovatchev et al. (2006) Evaluation of a New Measure of Blood Glucose Variability in,
#' Diabetes
#' \emph{Diabetes care} \strong{29} .2433-2438,
#' \doi{10.2337/dc06-1085}.
#'
#' @examples
#'
#' data(example_data_1_subject)
#' lbgi(example_data_1_subject)
#'
#' data(example_data_5_subject)
#' lbgi(example_data_5_subject)
#'
lbgi <- function(data){
fbg = gl = id = NULL
rm(list = c("fbg", "gl", "id"))
data = check_data_columns(data)
is_vector = attr(data, "is_vector")
out = data %>%
dplyr::filter(!is.na(gl)) %>%
dplyr::mutate(
fbg = log(gl)^{1.084} - 5.381,
fbg = pmin(fbg, 0)) %>%
dplyr::group_by(id) %>%
dplyr::summarise(
LBGI = 22.77 *
sum(fbg[ gl < 112.5]^2, na.rm = TRUE) /
sum(!is.na(gl))
)
if (is_vector) {
out$id = NULL
}
return(out)
}
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