Kidney Disease: What It Is, How It Is Found, and What Helps

Most advice about kidney health is backwards about one thing and silent about another.

The earlier version of this post ran to about 665 words across four disorders, with no citations, no numbers and no images. It listed one piece of prevention advice that would probably make kidney stones more likely.

It also never once mentioned a test. That omission matters more than any single tip in the article.

The kidneys sit at the back of the abdomen, one on each side, each roughly the size of a fist. They filter blood at about 180 litres a day and return about 99 percent of it. BruceBlaus, CC BY 3.0, via Wikimedia Commons

How kidney disease is actually found

Chronic kidney disease is defined precisely, and the definition is short enough to be useful.

The KDIGO guideline defines it as an abnormality in kidney structure or function that has been present for at least three months, with implications for health.

In practice that comes down to two ways of being diagnosed. Either your filtration rate has stayed low, or there is damage showing up in your urine.

A filtration rate below 60 for three months or more is chronic kidney disease on its own. And albumin in the urine counts as disease even when filtration looks normal.

That means two tests: a blood test for creatinine, from which the filtration rate is estimated, and a urine test for the albumin-to-creatinine ratio.

Why symptoms are the wrong thing to wait for

This is the fact the earlier version of this post inverted.

Chronic kidney disease is usually silent. Kidneys lose function gradually and they are built with a great deal of reserve, so a substantial loss can occur before anything is noticed.

Fatigue, ankle swelling, poor concentration and night-time urination are the symptoms usually listed, and the earlier version did list them. But it paired them with the line "life-threatening if not detected early."

That sentence is true and the symptom list makes it useless. You do not detect this early by feeling unwell. You detect it by taking a test.

A kidney on ultrasound. This is the general shape of how kidney disease is found: an image, a number, or a strip of paper, rather than a symptom. Ptrump16, CC BY-SA 4.0, via Wikimedia Commons

If you have diabetes, high blood pressure, or a family history of kidney disease, that test is worth having on a schedule and not waiting for.

What the number means

Figure	What it is
below 60	filtration rate, sustained, and you have CKD
3 months	how long the abnormality must persist to count
2 tests	blood creatinine, and urine albumin-to-creatinine ratio
2 of 3	about two thirds of CKD is caused by diabetes and blood pressure
180 litres	filtered by the kidneys each day
99 percent	of that volume reclaimed rather than lost

Figures from the sources listed at the foot of this article.

What the kidneys actually do

The filtration rate in that definition is not a metaphor. It is a measurement of something specific.

Human kidneys. The outer cortex does the filtering and the inner medulla concentrates the urine, which is how a very dilute filtrate becomes concentrated waste. Anatomist90, CC BY-SA 3.0, via Wikimedia Commons

A nephron, the working unit of the kidney. There are about a million per kidney. An estimated filtration rate is a number describing what the whole set is achieving together. Holly Fischer, CC BY 3.0, via Wikimedia Commons

Each nephron has a filtering end, a bundle of capillaries called a glomerulus, wrapped in a pressure-driven cup.

Filtration happens under pressure across the glomerulus, and the tubule then reclaims almost everything that passed through. Most of what returns to the blood is reclaimed rather than lost. Madhero88, CC BY 3.0, via Wikimedia Commons

Blood is pushed into that glomerulus at higher pressure than exists elsewhere, and water plus small solutes cross into the tubule while cells and proteins stay behind.

Then the tubule walks that filtrate back up and takes back nearly all of it, adjusting what it returns according to what the body needs at the moment.

So a healthy kidney makes about 180 litres of filtrate a day and returns roughly one to two percent of it as urine. The rest is reclaimed.

Why diabetes and blood pressure dominate

Two conditions account for the large majority of chronic kidney disease, and both damage the glomerulus.

Sustained high blood sugar changes the structure of the filtering capillaries. Sustained high blood pressure applies mechanical force to them. Neither gives any warning.

A blood pressure reading. Tracking this at home is the most useful thing most people at risk can do, because it is the leading modifiable cause of kidney disease. Steven Fruitsmaak, CC BY 3.0, via Wikimedia Commons

The earlier version of this post named both as causes, which was correct. It then gave no target and no follow-up.

It did correctly identify prolonged use of over-the-counter painkillers as a cause. Regular ibuprofen use is an under-recognised risk, and it is one people can stop.

Kidney stones, and the advice that goes the wrong way

About four in five kidney stones are calcium oxalate. Preventing them is mostly a question of urine chemistry, and the two levers that matter are fluid and calcium.

The error: do not cut calcium

The earlier version of this post listed high calcium and oxalate intake as a cause of stones, pointing at dairy and leafy greens, and advised cutting them.

That is backwards for calcium, and it would make stones more likely.

Calcium binds oxalate in the gut. That binding stops oxalate being absorbed, so less of it reaches the urine where it could crystallise onto a stone.

Remove the calcium and the oxalate is absorbed anyway. You lose the protection and keep the exposure.

Source	What it found
National Kidney Foundation	people with calcium kidney stones should not be on a low calcium diet
Curhan et al., NEJM 1993	no support for the belief that low calcium reduces stone risk
Sorensen et al. 2014	calcium restriction may increase stone formation via oxalate absorption
Mayo Clinic	about 1,200 mg of calcium a day is the preventive target

Dietary calcium and stone risk. The evidence points the opposite way to the earlier version of this post.

Leafy greens are a different food and a different problem. They are genuinely high in oxalate, and oxalate is worth moderating.

So the earlier version fused two foods that belong in opposite categories. Dairy was listed as harmful when it is protective. Leafy greens were listed as harmful for the wrong reason.

The right lever: urine volume

The earlier version advised at least eight glasses of water a day. That figure has no evidence behind it.

The quantity that reduces stone risk is not what you drink, it is what you excrete. The target used in renal nutrition practice is at least two and a half litres of urine a day.

A fixed glass count ignores body size, diet, activity and climate. Producing the urine is what matters, and you can be checked for it.

Two things the earlier version did get right here: cutting salt lowers the calcium in your urine, and cutting excess animal protein lowers the acidity that promotes certain stones.

The link between sodium and urinary calcium is direct. Roughly 2,300 milligrams of dietary sodium adds about 20 milligrams of calcium to the urine.

Urinary infections: what actually raises the risk

The earlier version listed poor hygiene as a cause, and specifically noted it increases risk in women.

That is the persistent myth, and it is worth replacing, because it causes shame rather than prevention.

Uropathogenic Escherichia coli adhering to bladder epithelium. The bacteria climb from the urethra into the bladder, which is why most urinary infections feel urgent rather than slow. Stefan Walkowski, CC BY-SA 4.0, via Wikimedia Commons

Most urinary infections start with bacteria from the bowel colonising the urethra and travelling upward. The things that predict it are anatomical and situational.

A shorter urethra, which is anatomy and not a habit. Sexual activity. Pregnancy. The changes after menopause. Diabetes. And catheters.

The specific advice to wipe front to back has been largely abandoned, because the anatomy, not the direction of wiping, is the dominant factor.

The symptom that does not help

The earlier version listed cloudy or strong-smelling urine as a symptom to watch for. Both are unreliable.

Cloudy urine is usually phosphate or a little mucus. A strong smell is often just what you ate. Neither tells you whether bacteria are present, and both send people to a clinic unnecessarily.

White blood cells in urine. This is the finding that actually indicates a urinary infection, and it is found on a dipstick or a culture rather than guessed from the way water looks. Bobjgalindo, CC BY-SA 4.0, via Wikimedia Commons

White cells in the urine are a better signal than the appearance of the urine itself.

Cranberry juice: narrower than it sounds

The advice to drink cranberry juice was stated as though it applied to everyone. It does not.

The 2023 Cochrane review finds cranberry reduces infections in women with recurrent infections, in children, and in people undergoing procedures. In the general population the benefit is small to absent.

Dose matters as well. A 2024 dose-response study found an effect only above roughly 36 milligrams of proanthocyanidins a day, which nobody can judge from a glass of juice.

Urinating after intercourse does reduce risk, and unlike the cranberry advice it is well supported.

When kidneys fail

End-stage kidney disease is not a mild version of the earlier stages. It means the kidneys have failed to a point where life cannot be sustained without replacement therapy.

That sentence is in the earlier version of this post, and it is its bluntest and most important line. It belongs near the top rather than at the end of a symptom list.

An ultrasound of a kidney in end-stage disease. Damage that looks this obvious on a scan is usually damage that had been progressing quietly for years. Kristoffer Lindskov Hansen, Michael Bachmann Nielsen and Carsten B. Larsen, CC BY 4.0, via Wikimedia Commons

There are two treatments, and both are significant.

Peritoneal dialysis, where fluid is cycled through the abdominal cavity to filter the blood. The alternative to a transplant is not nothing; it is a regimen performed every day. Manu5, CC BY-SA 4.0, via Wikimedia Commons

Dialysis does what the kidneys can no longer do. It is done daily and it works, but it treats the failure rather than reversing it.

A transplant replaces the organ. It is a major procedure and it requires lifelong medication, but it restores kidney function.

Two things worth correcting from the original

The listed causes of failure included severe infections leading to organ failure, which is vague and not the usual route.

The usual route is chronic kidney disease progressing, usually driven by diabetes or blood pressure. Acute kidney injury from severe illness can also destroy function, and it is worth knowing about.

The original also advised limiting caffeine as a way to protect the kidneys. Excess alcohol is a genuine risk. Caffeine is not established as damaging to kidneys at moderate intakes, and bundling it with alcohol misleads people about which habit to change.

Key Terms in Plain English

These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.

Term	What it means
Nephron	The filtering unit of the kidney. About a million per kidney.
Glomerulus	The tuft of capillaries where filtration happens, under higher pressure than anywhere else in the body.
Creatinine	A waste product measured in blood. Filtration rate is estimated from it.
eGFR	Estimated glomerular filtration rate. Below 60 for three months means chronic kidney disease.
Albuminuria	Albumin leaking into the urine. Present, it is kidney damage even when filtration looks normal.
Albumin-creatinine ratio	The urine test that detects albumin loss, corrected for how concentrated the sample is.
Oxalate	A compound found in many plants. It binds calcium in the gut, which is protective rather than harmful.
Nephrology	The medical specialty treating kidney disease.
End-stage kidney disease	Kidney failure requiring dialysis or a transplant to sustain life.
Dialysis	Filtering the blood artificially to do what the kidneys can no longer manage.
Pyuria	White blood cells in the urine, which indicate an inflammatory response in the urinary tract.

Terms used above, in roughly the order they first appear.

Key Takeaways

• Chronic kidney disease is defined as an eGFR below 60 for three months, or albumin in the urine. Two tests find it: creatinine and urine albumin.
• It is usually silent until much function is lost, so waiting for symptoms is the wrong screening strategy.
• Diabetes and high blood pressure cause about two thirds of cases. Both are modifiable, and neither gives warning.
• Do not cut calcium to prevent kidney stones. It binds oxalate in the gut, and restricting it increases risk.
• Leafy greens are a separate issue. They are high in oxalate and worth moderating.
• The stone target is urine output, at least 2.5 litres a day, not a count of glasses.
• Cranberry juice helps specific groups with recurrent infections. It does not reliably prevent infections in the general population.
• Cloudy or strong-smelling urine is not a test. White cells in the urine are a better signal.

Frequently Asked Questions

How do I know if I have kidney disease?

You need a test, not a symptom. Ask for a blood creatinine to get your filtration rate, and a urine albumin-to-creatinine ratio.

If you have diabetes, high blood pressure or a family history, ask about a schedule for these and write it down.

Should I stop eating dairy if I have had kidney stones?

No. This is the advice most often given and most often it should not be. Reducing calcium intake increases your risk.

Eat a normal amount of calcium, and take it with meals. What is worth reducing is sodium and excess animal protein.

How much water should I drink?

If you are trying to prevent stones, aim to produce at least two and a half litres of urine a day, and ask a clinician how to check.

If you already have reduced kidney function, large volumes can be harmful, so that advice should come from your clinician.

Does cranberry juice work for infections?

In specific groups, yes. Women with recurrent infections, children, and people around certain procedures show fewer infections.

For the general population the benefit is small and hard to measure, and the dose is not something you can judge from a glass.

Can kidney damage be undone?

No, and that is why early detection matters more than any lifestyle tip in this article.

Progression can be slowed substantially by controlling blood pressure and blood sugar. Loss of function that has occurred does not come back.

Key references
• KDIGO 2024 Clinical Practice Guideline for CKD
• UK Kidney: CKD staging
• National Kidney Foundation: calcium kidney stones
• Curhan et al., dietary calcium and kidney stones, NEJM 1993
• Calcium intake and urinary stone disease
• Mayo Clinic: kidney stones and calcium
• Cochrane: cranberries for preventing urinary tract infections
• Chronic kidney disease: diagnosis and management

Images: Wikimedia Commons, with the author and license named in each caption. Clinical figures follow the sources above.