nankivell_spk_US: eGFR using the Nankivell-SPK formula (US units)
Description
A wrapper function for the nankivell_spk() vectorised function to calculate the eGFR
using the Nankivell-SPK formula using serum creatinine in mg/dl.
To allow for serial measurements over time, such as
for transplant follow-up data, there is an optional offset = n parameter which increases the age
value used in the equation by n years. It has not been possible to run automated tests
to confirm the
accuracy of this function, so please use with caution.