Glomerular filtration rate (GFR) measures the ability of the kidneys to clear wastes. Glomeruli are tiny clusters of blood vessels in the kidney. As the blood flows through them, the water and chemicals in it get squeezed out into the tubules of the kidney, leaving behind cells, proteins, and a tiny amount of water. As this filtered liquid goes through the tubules, most of it (95-98%) gets taken up by tubular cells and returned to the body. Under normal conditions, an adult produces 144 liters of filtrate each day, but puts out less than 2 liters of urine.
To measure GFR we need a substance that is freely filtered in the glomeruli and not altered by the tubules of the kidney. Inulin and other manufactured substances can measure GFR most accurately, but these methods require continuous intravenous drips. They are inconvenient and expensive. Doctors and scientists looked for a chemical within the body that met these criteria. They identified creatinine, a muscle protein now included on most routine biochemical panels.
Measuring clearance of creatinine requires a measurement of the blood level, a timed urine collection, and measurement of the creatinine in the urine. Collecting urine over a given period of time can be annoying. Thus began the search for a way to estimate GFR from just a blood test.
Since muscles make creatinine, people with more muscle have higher levels regardless of kidney function. In pediatrics, where patients normally change size over time, these issues cause even more problems. The first pediatric formula, developed by Schwartz et al in the 1970s, used height as a proxy measure of muscle mass. Over time, this equation has been refined as our methodology to measure creatinine has improved. Several large studies of chronic kidney disease in adults led to other equations (see table).
The question remained when to switch from the pediatric formula to one of the adult equations. Selistre et al recently looked at correlation and agreement among these calculated values and measured inulin clearance (that gold standard) in adolescents and young adults from 10 to 25 years of age. Subjects had kidney function ranging from stage 1 (normal) to stage 4-5 (<20% of normal) in all age groups.
They found that the Schwartz 2009 equation provided the most accurate estimation of true GFR, across all age groups and all ranges of kidney function. This formula tended to underestimate GFR in those with normal function, but provided good agreement in other categories. The adult equations tended to overestimate GFR by up to 30%.
As the population of patients with chronic kidney disease grows, we need convenient ways to track kidney function over time. This study fills a gap, telling us the best way to do this in adolescents and young adults.
At least for now.