DETERMINATION OF THE QUANTITY OF SECRETING TISSUE IN THE LIVING KIDNEY

Which is the best glomerular filtration marker: Creatinine, cystatin C or both?

BACKGROUND

The glomerular filtration rate (GFR) is estimated by the serum or plasma concentration of creatinine and/or cystatin C using equations that include demographic data. The equations worldwide most widely used are those of the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) consortium and updated in 2021 to remove the Afro-American racial correction factor. In 2021 and then in 2023, the European Kidney Function Consortium also developed equations based on creatinine and cystatin C, usable across the full age spectrum, and constructed by including the Q value (i.e. the median creatinine or cystatin C in healthy men and women, which is customizable for specific populations).

METHODS

The aim of this narrative review is to examine the strengths and weaknesses of each biomarker.

RESULTS

Both biomarkers have non-GFR determinants, namely muscle mass, protein intake and tubular secretion for creatinine; dysthyroidism and systemic corticosteroids for cystatin C, as well as other more debated determinants (diabetes, obesity, proteinuria, inflammatory syndrome). These non-GFR determinants are the reason why no equation based on a single endogenous biomarker has an accuracy within 30% greater than 90% over the entire age spectrum (in at least one patient in 10, estimated GFR is at least 30% higher or at least 30% lower than the measured GFR).

CONCLUSION

Equations combining the two biomarkers provide a better estimate of GFR, particularly in the subgroup of patients whose estimates based on each of the biomarkers are highly discordant. These patients must also be identified as being at increased risk of morbidity, particularly cardiovascular, and mortality.

THE EXCRETION OF CYANOL, AZOFUCHSIN I AND WATER BY THE KIDNEYS OF RABBITS

1. The excretion of water has been studied in a large number of experiments on rabbits. After the ingestion of 100 cc. of water, the day before the experiment and as part of the experiment, the average diuresis amounted to 0.6 cc. per minute during a half hour period. The highest individual rate was 1.5 cc. per minute. 2. The excretion of cyanol and of azofuchsin I has also been studied. It has been shown that the cyanol excretion curve closely parallels the inulin excretion curve of Kaplan and Smith. Evidence is presented that cyanol is disposed of entirely or mainly by filtration. 3. The excretion of azofuchsin I is not only very different from that of cyanol or inulin, but almost identical with that of phenol red. Evidence is presented that at low plasma concentration azofuchsin is, in the main, secreted.