Glomerular filtration is the main life sustaining kidney function, and kidney stones can cause enough damage to lower it. Usually the reduction is very modest, but sometimes stones can cause kidney failure. This means, like all diseases, stones are best prevented as early and as completely as possible.
This ‘just the facts’ version tells about what filtration is, how physicians measure it, how much kidney stones lower it, and how often that occurs.
Much of this text is redacted from the other two articles. I left out pictures, data, and links to PubMed to emphasize the main points.
The featured painting is by You Si 游思 (b. 1954), based in Shanghai, China. He is the son of two artists who lived through the cultural revolution and paints in new ways. I think his work is gorgeous but could not find a serious biography or critical study, or any museums that show his work.
What Filtration Is
Our two kidneys are each made up of about one million nephron units. A nephron is a complete and complex structure that begins its function by filtering from blood plasma a clear solution of water and small molecules. That filtrate enters the long loopy tubule of the nephron that reabsorbs a vast fraction of water and salts back into the blood leaving behind only that destined for export out of the body.
The glomerulus, a tuft of capillaries nestled in a tiny cup that drains into its tubule, performs this filtration. So we name it glomerular filtration rate, or GFR.
Blood flow and pressure from the heart drive filtration, but kidneys can regulate its rate. The cells that line the tubules regulate what fractions of innumerable individual salts and small molecules are taken back or let go into the urine.
Filtration Keeps Us Alive
Filtration rids the blood plasma of metabolic wastes, like oxalate, and permits us to excrete the extra water, salt, and all other materials we might eat or drink every day. Without filtration we need dialysis or transplantation to continue living. So this kidney function stands above all the others. When physicians speak of kidney function, they mean, and measure, filtration.
Filtration and Kidney Stones
Kidneys filter copious volumes of water and small molecules – like calcium, oxalate, citrate, phosphate, uric acid, sodium – hundreds of distinct materials. They then return most of this flood – 100 to 200 liters a day of water alone – back into the blood keeping behind only what they destine for export as urine.
Because kidneys filter such vast amounts, very subtle variations in what they take back into blood can load the urine with stone forming salts like calcium and oxalate, and cause stones. In other words, stones arise in significant measure because of subtle imbalances between filtration and reabsorption of key materials like calcium, oxalate, citrate, and water itself.
The filtering system, though powerful, is delicate. Through obstruction, infection, loss of one kidney, or surgical trauma stones can injure it. Such injuries reduce the ability of kidneys to protect the body, and when severe can lead to chronic kidney disease and even need for dialysis or transplantation.
How We Measure Glomerular Filtration
Muscles make creatinine steadily and send it into the blood. Like almost all small molecules, kidneys filter it. Serum creatinine therefore reflects the balance between muscle production and filtration. When filtration falls serum creatinine rises. It is like your golf score – bad day, higher score. Serum creatinine gauges filtration adequately enough for clinical practice. But because kidneys not only filter creatinine but also secrete it from blood into urine, creatinine overestimates true filtration.
To get a better estimate, scientists compared serum creatinine to true GFR in reasonably large populations and calculated the best fit between them using regression techniques. The resulting regression equation gives estimated GFR, or eGFR, from serum creatinine, age, sex, and race. This highly derived index dominates medical practice. When physicians speak of reduced kidney function they mean reduced eGFR.
Kidney Function in Stone Formers
Average Function is modestly reduced
Our Own Patients
In our own patients, we found that stone formers have lower kidney function than normal people. The common idiopathic calcium stone formers are just slightly low. Likewise for those with primary hyperparathyroidism. But kidney function is reduced by about 20% in people who make stones because of bowel disease or diseases like renal tubular acidosis or cystinuria. Uric acid and struvite stone formers have even greater reductions.
US Health Surveys
The national NHANES data survey results showed modestly reduced kidney function in overweight people who had a history of stones. Women who formed stones had increased risk for serious chronic kidney disease, and even dialysis.
This community uses Mayo Clinic for care so records are very complete. A history of kidney stones increased risk of CKD and eventual dialysis.
Two extensive reviews of all published studies – not just the few I mentioned – concluded that where ever looked for, stone forming raised risk of high blood pressure, serious chronic kidney disease, and even dialysis.
Though Increased, Kidney Disease Rates are Not High
Stones raise the risk of serious kidney disease from the very low levels in the general population to higher though still low levels. The vast majority of stone formers do well and never have problems that require special kidney care. Only rarely do stone formers need dialysis. But they can come to need such care more frequently than people who never formed a stone.
The message is obvious. Stone recurrence should be minimized through prevention efforts. Physicians who practice stone prevention need to measure eGFR appropriately, and if it begins to fall take proper steps to mitigate the causes of that fall.
Obstruction from stones always needs prompt, skilled, urological care that minimizes risk and trauma from surgery. Because silent obstruction can destroy a kidney, any symptoms that suggest new stone passage deserve urological attention.
High Blood Pressure
The NHANES and Olmsted County data show that a history of stones raises risk of elevated blood pressure. Loss of kidney function, even subtle, could raise blood pressure, and increased blood pressure can harm kidneys and reduce GFR. Unlike significant kidney disease, high blood pressure is easily treated to prevent strokes, heart disease, and kidney failure. But high blood pressure commonly escapes diagnosis and adequate treatment.
This means kidney stone prevention clinics are high blood pressure clinics, or at least a common place where high blood pressure can be first detected. All stone formers need to know their blood pressure and seek care if it is elevated.
Stones are a painful and recurrent chronic disease that kidneys generally tolerate with at most a modest amount of injury and function loss. But compared to people without stones, those who make them have measurably increased risk of high blood pressure and loss of kidney function. Years of study have made clear that this disease demands attentive treatments aimed at prevention, and that surgeons who manage stones do everything possible to preserve kidney tissue.