What Is Chronic Kidney Disease? Stages, Causes, and Protecting Kidney Function

What Is Chronic Kidney Disease?

Chronic kidney disease (CKD) is a condition in which the kidneys gradually lose their ability to filter waste products and excess fluids from the blood over months to years. Unlike acute kidney injury (which can reverse), CKD represents permanent, progressive damage that accumulates over time. It affects approximately 37 million Americans (1 in 7 adults) and is a leading cause of morbidity, often progressing silently until significant function has been lost.

How the Kidneys Work

The kidneys are two bean-shaped organs that perform critical functions:

• Filter approximately 200 liters of blood per day, removing waste products and excess fluid as urine
• Regulate blood pressure via the renin-angiotensin system
• Produce erythropoietin (EPO) — a hormone that stimulates red blood cell production
• Activate vitamin D for bone health
• Maintain electrolyte balance (sodium, potassium, phosphate, calcium)

When kidneys are damaged, all of these functions are impaired — explaining CKD's wide-ranging effects on cardiovascular health, bone density, anemia, and more.

Causes and Risk Factors

The two leading causes account for 75% of CKD cases:

• Diabetes (diabetic nephropathy): Chronically high blood sugar damages the delicate filtration units (glomeruli) of the kidney. Diabetic nephropathy is the leading cause of kidney failure requiring dialysis in the U.S.
• Hypertension: High blood pressure damages the kidney's blood vessels, impairing filtration and concentrating function, while damaged kidneys worsen blood pressure control — a vicious cycle.

Other causes: glomerulonephritis (immune-mediated inflammation), polycystic kidney disease (genetic), recurrent kidney infections (pyelonephritis), long-term NSAID use, certain chemotherapy agents, and rare conditions like IgA nephropathy and lupus nephritis.

Stages of CKD (by GFR)

CKD is staged by glomerular filtration rate (GFR) — the rate at which the kidneys filter blood, measured in mL/min/1.73m²:

• Stage 1: GFR ≥ 90 — Normal or high GFR with kidney damage markers (protein in urine)
• Stage 2: GFR 60–89 — Mildly decreased
• Stage 3a/3b: GFR 30–59 — Moderately decreased; symptoms may begin (fatigue, fluid retention, mild anemia)
• Stage 4: GFR 15–29 — Severely decreased; uremia symptoms (nausea, weakness); preparation for dialysis/transplant begins
• Stage 5 (Kidney Failure / ESRD): GFR < 15 — Requires dialysis or kidney transplant to survive

Symptoms: The Silent Progression

CKD is often called a "silent killer" because symptoms are absent or nonspecific until stage 4–5. Early warning signs when they appear: foamy urine (protein), blood in urine, swelling of legs/ankles (edema), elevated blood pressure, fatigue (from anemia), and decreased urine output. Most patients are diagnosed through routine blood and urine tests — another reason regular screening for high-risk patients (diabetics, hypertensives) is essential.

Slowing CKD Progression

• Blood pressure control: Target < 130/80 mmHg. ACE inhibitors and ARBs provide additional kidney-protective effects beyond blood pressure reduction.
• Blood sugar control in diabetes: HbA1c < 7% significantly reduces progression. SGLT2 inhibitors (empagliflozin, dapagliflozin) have demonstrated kidney-protective effects independent of glucose lowering — a major advance.
• Diet: Low-sodium diet for blood pressure; limiting dietary potassium and phosphate in later stages; adequate but not excessive protein (excessive protein increases kidney workload)
• Avoid nephrotoxic agents: NSAIDs (ibuprofen, naproxen) should be avoided or minimized; iodinated contrast agents require precautions; certain antibiotics require dose adjustment
• Smoking cessation: Smoking accelerates CKD progression