Burkart J et al
An elevated ratio of measured to predicted creatinine production
in CAPD patients is not a sensitive predictor of non-compliance
with the dialysis prescription
Perit Dial Int
(Mar) 16:142-146 1996

Also reviewed is Comparison of measured and predicted creatinine excretion
is an unreliable index of compliance in PD patients Blake et al, Perit
Dial Int 16:147-153, 1996 (same issue).
The two manuscripts in the March issue of PDI deal with the topic of
non-compliance in PD patients and, in particular, with the use of
creatinine excretion ratios to detect this important problem. Three
years ago Keen et al suggested that non-compliance could be suspected
in patients whose measured creatinine excretion was significantly
higher than their predicted creatinine excretion. Measured creatinine
excretion is a combination of dialysate and urinary creatinine losses
combined with an estimate of extra-renal creatinine degradation based
on the Mitch-Walser formula. Predicted creatinine excretion is based
on the Cockroft-Gault formula. The creatinine excretion ratio is the
measured divided by the predicted creatinine excretion. The theory is
that in non-compliant patients creatinine accumulates when they are
non-compliant but that on the day of the collection, when they are
compliant, this accumulated creatinine is washed out. A creatinine
excretion ratio of more than 1.24 has been proposed as a marker of
non-compliance. Using this methodology, Keen et al, Warren et al and
Nolph et al all suggested that non- compliance is quite common in CAPD
patients, with a prevalence as high as 26%.
The two articles in the March 1996 PDI, however, raise questions about
this methodology. Burkart et al show, in a study of nine patients on
CAPD, that creatinine excretion does indeed increase significantly
after a three day period of intentional non-compliance. They also
demonstrate, however, that at baseline three of their patients have
very low creatinine excretion ratios and four have high creatinine
excretion ratios. In the three with low ratios, the value still
remains below unity, even after the intentional non-compliance has
taken place. Thus, these patients would not be picked up in a survey
of non-compliance. Conversely, four of their patients had a baseline
ratio that was >1.24. This suggests that these patients were either
non- compliant before the study began or else that the ratio is
unreliable.
The second article in the same edition, from our group, shows that in
43 patients the baseline ratio averaged 1.13 and that 40% had a ratio
higher than the suggested cutoff of 1.24. Because these patients
denied non-compliance, six of them were investigated more closely and
had the ratio repeatedly measured during a four day period of
guaranteed, observed compliance. The ratios did not change during
that time, suggesting that these patients had high ratios due, not to
non-compliance, but rather to higher creatinine excretions than the
Cockroft-Gault formula predicted. In one case, where non-compliance
was admitted, the ratio was only 1.09.
Comment: These findings suggest that the creatinine
excretion ratio is neither a sensitive nor a specific indicator of
non-compliance. It also suggests that existing literature based on
this technique may not be reliable. We need a better technique to
measure non-compliance. One possibility that is suggested in both
papers, and in the accompanying editorial by Brandes, is that serial
creatinine excretion measurements may be helpful. Thus, a rise in
creatinine excretion ratio over time would suggest that the patient is
either non-compliant or is increasing his lean body mass. If the high
ratio is associated with other features of better nutrition, this
might suggest lean body mass is increasing. If, in contrast, the
higher ratio is associated with signs of poor nutrition, this might
suggest that the problem is non-compliance. This might be the case
even in cases where the absolute level of the creatinine excretion
ratio was < 1.0; i.e., the increase might be, for example, from 0.7
to 0.85. Thus, there is still a case to be made for serial follow-ups
of creatinine excretion ratios, but, as with many of such tests,
clinical judgement will be needed to interpret them properly.
(Peter G. Blake, M.D., Victoria Hospital, London, Ontario)