  Introduction: Kidney function is reported using estimates of glomerular
filtration rate (eGFR). However, eGFR values are recorded without reference to
the creatinine (SCr) assays used to derive them, and newer assays were
introduced at different time points across laboratories in UK. These changes
may cause systematic bias in eGFR reported in routinely collected data; even
though laboratory reported eGFR values have a correction factor applied.
  Design: An algorithm to detect changes in SCr which affect eGFR calculation
method by comparing the mapping of SCr values on to eGFR values across a
time-series of paired eGFR and SCr measurements.
  Setting: Routinely collected primary care data from 20,000 people with the
richest renal function data from the Quality Improvement in Chronic Kidney
Disease (QICKD) trial.
  Results: The algorithm identified a change in eGFR calculation method in 80
(63%) of the 127 included practices. This change was identified in 4,736
(23.7%) patient time series analysed. This change in calibration method was
found to cause a significant step change in reported eGFR values producing a
systematic bias. eGFR values could not be recalibrated by applying the
Modification of Diet in Renal Disease (MDRD) equation to the laboratory
reported SCr values.
  Conclusions: This algorithm can identify laboratory changes in eGFR
calculation methods and changes in SCr assay. Failure to account for these
changes may misconstrue renal function changes over time. Researchers using
routine eGFR data should account for these effects.
