Acute glomerulonephritis is a sudden inflammation of the glomeruli, the tiny filtering units inside your kidneys. When these filters become inflamed, they lose the ability to properly clean your blood, leading to a buildup of fluid, waste, and protein or blood leaking into your urine. It most commonly follows a bacterial infection, particularly strep throat or a strep skin infection, and it affects children more often than adults.
How the Kidneys Become Inflamed
Each kidney contains roughly one million glomeruli, which are clusters of miniature blood vessels that filter waste and excess water from your bloodstream. In acute glomerulonephritis, the immune system misfires. Instead of just fighting off an infection, it produces immune complexes (clumps of antibodies bound to proteins from the invading bacteria) that get trapped in the walls of these tiny filters.
Once lodged there, these immune complexes trigger a chain reaction. The body’s inflammatory and clotting systems activate, releasing signaling molecules that cause the cells lining the glomeruli to multiply and swell. The filter walls thicken, narrowing the passages that blood normally flows through. The result is a sharp drop in the kidneys’ filtration rate. When less fluid is filtered, the body retains salt and water, which drives up blood pressure and causes swelling.
The Most Common Trigger: Strep Infections
The classic cause is a prior infection with specific strains of group A streptococcus, the same bacterium behind strep throat and impetigo. Not every strep strain triggers kidney inflammation. Only certain “nephrogenic” strains have surface proteins that generate the immune complexes capable of lodging in glomeruli.
The timing is distinctive and helps doctors connect the dots. After a strep throat infection, kidney symptoms typically appear 1 to 2 weeks later. After a strep skin infection like impetigo, the delay is longer, roughly 6 weeks. This gap exists because it takes time for the immune system to build up enough antibody-antigen complexes to cause visible kidney damage. Other infections, including certain viral, bacterial, and parasitic illnesses, can also trigger acute glomerulonephritis, but post-streptococcal disease remains the most recognized form worldwide.
Recognizable Symptoms
Acute glomerulonephritis often announces itself over just a few days with a cluster of symptoms that can feel alarming:
- Dark or cola-colored urine. Blood leaking through the damaged filters gives urine a brownish or tea-like color. Sometimes the blood is only visible under a microscope.
- Swelling (edema). Puffy eyelids in the morning are a hallmark, especially in children. As the day goes on, swelling often shifts to the ankles and lower legs. In severe cases, fluid retention can become widespread.
- High blood pressure. Salt and water retention raises blood volume, pushing blood pressure above normal levels.
- Reduced urine output. Because the kidneys are filtering less, you may notice you’re urinating less frequently or producing smaller amounts.
- Foamy urine. Protein slipping through the damaged filters can make urine appear frothy.
- Weight gain. Rapid weight gain over days, driven by fluid retention rather than fat, is common.
Some people also feel generally unwell, with fatigue, mild nausea, or a dull ache in the flank area over the kidneys. In children, a parent may first notice the puffy face in the morning and dark urine.
How It Differs From Nephrotic Syndrome
Both acute glomerulonephritis and nephrotic syndrome involve the kidney filters, but they look different clinically. Acute glomerulonephritis (sometimes called nephritic syndrome) is defined by blood in the urine, elevated blood pressure, and reduced kidney function. Protein does leak into the urine, but not in massive quantities. Nephrotic syndrome, by contrast, involves very heavy protein loss in the urine, severe swelling, high cholesterol, and typically no blood in the urine or blood pressure elevation early on.
In practice, the distinction matters because the causes, treatments, and outlook differ. Seeing blood in the urine alongside swelling and high blood pressure points toward a nephritic process. Seeing dramatic, generalized swelling with foamy urine but no blood points more toward nephrotic syndrome. Some conditions can overlap, but the initial pattern helps guide diagnosis.
What Happens During Diagnosis
A doctor suspecting acute glomerulonephritis will typically start with a urine sample. Under a microscope, the urine often shows red blood cells and, more specifically, red blood cell casts, which are clusters of blood cells molded into the shape of the kidney’s tiny tubules. These casts are a strong signal that the bleeding is coming from the glomeruli rather than somewhere else in the urinary tract, like the bladder.
Blood tests check how well the kidneys are filtering by measuring waste products that build up when filtration drops. Complement levels, part of the immune system’s toolkit, are often low during the acute phase because they’re being consumed in the inflammatory reaction inside the kidneys. If a strep infection is suspected, tests can detect antibodies the body made against streptococcal proteins, confirming a recent infection even if the sore throat or skin sores have already healed. In some cases, a kidney biopsy (removing a tiny sample of kidney tissue with a needle) is needed to confirm the type and severity of inflammation.
Treatment and What to Expect
For the most common form, post-streptococcal glomerulonephritis, there is no medication that directly reverses the kidney inflammation. Treatment focuses on managing symptoms while the immune reaction runs its course. This typically means controlling blood pressure, reducing fluid overload with medications that help the kidneys excrete excess salt and water, and restricting dietary sodium to prevent further fluid buildup. If a strep infection is still active, antibiotics clear the bacteria, though this doesn’t reverse the kidney inflammation already underway.
In more severe or non-strep-related forms of acute glomerulonephritis, treatment may include medications that suppress the overactive immune response. The specific approach depends on the underlying cause identified through biopsy and blood work.
During the acute phase, you can expect to have your blood pressure, weight, and urine output monitored closely. Fluid and salt intake are usually limited. Most people feel noticeably better within one to two weeks as swelling goes down and urine color returns to normal, though lab abnormalities can linger longer.
Recovery and Long-Term Outlook
Children with post-streptococcal glomerulonephritis have an excellent prognosis. The vast majority recover full kidney function within weeks to months. Blood in the urine usually resolves within a few weeks, though trace amounts of protein in the urine can persist for months and still represent a normal recovery trajectory.
Adults tend to have a less predictable course. Recovery is still common, but the risk of lingering kidney damage or progression to chronic kidney disease is higher than in children. Factors that signal a worse outlook include severe kidney impairment at the time of diagnosis, heavy protein loss in the urine, and older age.
For non-strep causes of acute glomerulonephritis, the outlook varies widely depending on the specific disease driving the inflammation. Some forms respond well to immune-suppressing treatment, while others, particularly rapidly progressive types, can cause permanent kidney damage within weeks if not treated aggressively. This is why identifying the cause matters so much. A kidney biopsy often provides the clearest answer when the clinical picture is ambiguous or the disease isn’t following the expected post-strep pattern.
Preventing Recurrence
Post-streptococcal glomerulonephritis rarely recurs. Once the immune reaction resolves, you are not at increased risk for a second episode, even if you get another strep infection later. Prompt treatment of strep throat with antibiotics may reduce the chance of developing glomerulonephritis in the first place, though this hasn’t been definitively proven for skin infections. Good hygiene practices, particularly hand washing and proper wound care, help prevent the spread of strep bacteria, especially in crowded living conditions where outbreaks are more common.