nankivell_spk: eGFR using the Nankivell-SPK formula
Description
A vectorised function to calculate the eGFR using the Nankivell-SPK formula. By default the
equation accepts serum creatinine in <U+00B5>mol/l but can be changed to
mg/dl by setting the units parameter to "US". 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.