roc {iglu}

Calculate the Rate of Change at each time point (ROC)

Description

The function roc produces rate of change values in a tibble object.

Usage

roc(data, timelag = 15, dt0 = NULL, inter_gap = 45, tz = "")

Arguments

data

DataFrame object with column names "id", "time", and "gl".

timelag

Integer indicating the time period (# minutes) over which rate of change is calculated. Default is 15, e.g. rate of change is the change in glucose over the past 15 minutes divided by 15.

dt0

The time frequency for interpolation in minutes, the default will match the CGM meter's frequency (e.g. 5 min for Dexcom).

inter_gap

The maximum allowable gap (in minutes) for interpolation. The values will not be interpolated between the glucose measurements that are more than inter_gap minutes apart. The default value is 45 min.

tz

A character string specifying the time zone to be used. System-specific (see as.POSIXct), but " " is the current time zone, and "GMT" is UTC (Universal Time, Coordinated). Invalid values are most commonly treated as UTC, on some platforms with a warning.

Details

A tibble object with a column for subject id and a column for ROC values is returned. A ROC value is returned for each time point for all the subjects. Thus multiple rows are returned for each subject. If the rate of change cannot be calculated, the function will return NA for that point.

The glucose values are linearly interpolated over a time grid starting at the beginning of the first day of data and ending on the last day of data. Because of this, there may be many NAs at the beginning and the end of the roc values for each subject. These NAs are a result of interpolated time points that do not have recorded glucose values near them because recording had either not yet begun for the day or had already ended.

The ROC is calculated as \frac{G(t_i) - G(t_{i-1})}{t_i - t_{i-1}} where G_i is the Glucose measurement at time t_i and G_{i-1} is the Glucose measurement at time t_{i-1}. The time difference between the points, t_i - t_{i-1}, is selectable and set at a default of 15 minutes.

Value

A tibble object with two columns: subject id and rate of change values

Author(s)

Elizabeth Chun, David Buchanan

References

Clarke et al. (2009) Statistical Tools to Analyze Continuous Glucose Monitor Data, Diabetes Diabetes Technology and Therapeutics 11 S45-S54, doi:10.1089/dia.2008.0138.

Examples

data(example_data_1_subject)
roc(example_data_1_subject)
roc(example_data_1_subject, timelag = 10)

data(example_data_5_subject)
roc(example_data_5_subject)