Two novel GFR equations predict kidney function in elderly

By Lauretta Ihonor, medwireNews reporter

Researchers report that two "new" equations may accurately measure kidney function in elderly individuals with nonsevere kidney disease.

Both equations, one based on serum creatinine (Berlin Initiative Study [BIS]1 equation) and the other on creatinine plus cystatin C measurements (BIS2 equation), show strong a correlation with measured glomerular filtration rate (mGFR), explain Elke Schaeffner (Charité Hospital, Berlin, Germany) and co-investigators.

They add: "This is important because a validated equation to estimate GFR in older adults, especially in cases of normal or only moderately reduced kidney function, has been lacking."

As reported in the Annals of Internal Medicine, the validity of the BIS1 and BIS2 equations was tested among a group of 610 individuals aged 70 years or older with no or mild-to-moderately reduced kidney function (GFR <60 mL/min per 1.73 m²).

All patients underwent estimated GFR (eGFR) calculation using the BIS1 and BIS2 equations in addition to conventional eGFR equations, such as Cockcroft‑Gault (adjusted for body surface area), abbreviated Modification of Diet in Renal Disease (MDRD), and Chronic Kidney Disease Epidemiology (CKD-EPI).

These eGFRs were then compared with mGFR, obtained using iohexol clearance values.

Schaeffner and colleagues found that of all eGFR values obtained, those calculated with the BIS2 equation showed the greatest similarity to mGFR, with a mean eGFR to mGFR difference (bias) of 0.09.

This was closely followed by the BIS1 and then the Cockcroft‑Gault equations, with respective mean biases of 0.11 and 2.74.

All other equations markedly overestimated GFR.

In light of this, Schaeffner and team advise that "the BIS2 equation should be used to estimate GFR in persons aged 70 years or older with no or mild to moderately reduced kidney function."

They add: "If cystatin C is not available, the BIS1 equation is an acceptable alternative."

The researchers also made observations that suggest that the muscle mass lost in old age compromises the accuracy of conventional eGFR equations, such as the MDRD equation, when used among elderly patients.

Schaeffner and team conclude that use of the BIS equations may help to minimize the likelihood of inaccurate staging of kidney disease among the elderly.

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