Renal tubular acidosis (RTA) is a condition where your kidneys fail to properly remove acid from your blood into your urine, causing your blood to become too acidic. Unlike most kidney diseases, RTA doesn’t typically show up as poor kidney filtration on standard tests. Instead, the problem lies in the tiny tubes (tubules) that fine-tune what stays in your blood and what gets flushed out. There are several distinct types, each involving a different part of the kidney’s acid-handling system.
How Your Kidneys Normally Handle Acid
Your body constantly produces acid as a byproduct of metabolism. To keep blood pH in a safe range, your kidneys do two things: they reclaim bicarbonate (your body’s main acid buffer) from the fluid being filtered, and they pump hydrogen ions (acid) into the urine. Normally, 85% to 90% of bicarbonate gets reabsorbed in the first stretch of the kidney tubule, called the proximal tubule. The remaining 10% is reclaimed further down in the distal tubule, where the final acidification of urine also happens.
When either of these processes breaks down, bicarbonate leaks out of the body or acid accumulates in the blood. The result is a type of metabolic acidosis, meaning your blood chemistry shifts toward the acidic end. RTA is classified by which part of the tubule is malfunctioning.
Type 1: Distal RTA
Type 1 is the most commonly discussed form. The defect sits in the distal tubule, where the kidney is supposed to secrete hydrogen ions into the urine. Because acid can’t be pumped out effectively, urine stays inappropriately alkaline, with a pH above 6 even when the blood is acidic. In a healthy kidney, urine pH would drop well below 5.5 under those circumstances.
Type 1 RTA can be inherited or acquired. Inherited forms involve mutations in genes that build the acid-pumping machinery of the distal tubule, including ATP6V0A4, ATP6V1B1, and SLC4A1. These genetic forms often appear in infancy or early childhood. Acquired forms develop later in life and are most frequently linked to Sjögren syndrome, an autoimmune condition that attacks moisture-producing glands. Systemic lupus, sickle cell anemia, chronic urinary tract obstruction, liver cirrhosis, and certain medications (lithium, amphotericin B) can also cause it.
A hallmark complication of type 1 RTA is kidney stones and nephrocalcinosis, a condition where calcium deposits build up inside the kidney tissue. Two factors drive this: chronic acidosis pulls calcium from bones and increases calcium in the urine, while at the same time the kidney reabsorbs more citrate than normal. Citrate normally acts as a natural inhibitor of stone formation, so low urinary citrate combined with high urinary calcium creates an ideal environment for stones.
Type 2: Proximal RTA
In type 2 RTA, the problem is upstream. The proximal tubule fails to reclaim its usual share of bicarbonate, so large amounts of this buffer spill into the urine and are lost. Once blood bicarbonate drops low enough, the reduced amount being filtered can finally be reabsorbed, and a new, lower equilibrium is reached. Above that threshold, bicarbonate floods into the urine and raises urine pH. Below it, urine can actually become acidic because the distal tubule still works normally.
Type 2 is rarer than type 1 and often appears alongside Fanconi syndrome, a broader proximal tubule dysfunction where the kidney also wastes glucose, phosphate, uric acid, and amino acids into the urine. Certain medications, particularly the antiviral tenofovir and the chemotherapy drug ifosfamide, can trigger Fanconi syndrome and proximal RTA together. Kidney stones are less common in type 2 than type 1 because the proximal tubule defect actually increases citrate in the urine, partially protecting against stone formation.
Children with untreated type 2 RTA are at risk for rickets (softening and weakening of bones) due to phosphate wasting, along with growth delays.
Type 4: Hyperkalemic RTA
Type 4 is the most common form of RTA overall and differs from the others in one critical way: it causes high potassium levels rather than low ones. The underlying problem is a deficiency of, or resistance to, aldosterone, a hormone that tells the kidney to retain sodium while excreting potassium and acid. Without adequate aldosterone signaling, potassium builds up in the blood, and acid secretion in the distal tubule slows down.
Diabetes-related kidney damage is the single most frequent cause. Other triggers include adrenal gland disorders and medications that interfere with aldosterone, such as certain blood pressure drugs. Because the acidosis in type 4 RTA is usually mild, treatment often focuses on managing the elevated potassium and addressing the underlying condition rather than giving bicarbonate supplements.
Type 3: A Rare Combined Form
Type 3 RTA is extremely rare and results from a deficiency of carbonic anhydrase II, an enzyme needed in both the proximal and distal tubules to generate the hydrogen ions and bicarbonate involved in acid-base balance. It combines features of types 1 and 2. The classic presentation is a triad of RTA, abnormally dense but brittle bones (osteopetrosis), and calcium deposits in the brain. Short stature, intellectual disability, and anemia may also occur.
Symptoms and How RTA Feels
Mild RTA can go unnoticed for years. When symptoms do appear, they tend to be nonspecific: fatigue, muscle weakness, and a general sense of feeling unwell. Low potassium (in types 1 and 2) can cause muscle cramps, weakness, or irregular heartbeat. High potassium (in type 4) can produce similar cardiac symptoms but through a different mechanism.
In children, the most visible sign is often poor growth. Chronic acidosis interferes with growth hormone action and pulls minerals from developing bones. Growth delays and bone disease (rickets in type 2, metabolic bone disease in type 1) can be the first clue that something is wrong. Adults may first come to attention through recurrent kidney stones or an incidental finding of nephrocalcinosis on imaging.
How RTA Is Diagnosed
The initial clues come from routine blood work showing a low bicarbonate level with a normal anion gap, a pattern called non-anion-gap metabolic acidosis. This narrows the possibilities to either RTA or gastrointestinal bicarbonate loss (such as from chronic diarrhea). A urine anion gap test helps distinguish between the two: in RTA, the kidney fails to excrete enough ammonium, producing a positive urine anion gap.
Once RTA is suspected, urine pH helps separate the types. A urine pH that stays above 5.5 despite blood acidosis points toward distal (type 1) RTA. In proximal (type 2) RTA, urine pH can drop below 5.5 once blood bicarbonate falls below the reabsorption threshold, making it trickier to catch on a single measurement. The combination of low urinary citrate (below 320 mg per 24 hours), a 24-hour urine pH above 6.2, and blood bicarbonate above 21 mEq/L has good diagnostic accuracy for incomplete distal RTA, a milder form that shows up during workups for kidney stones.
When the diagnosis is unclear, a provocative test can force the kidney to show its hand. The gold standard is the ammonium chloride loading test: the patient swallows an acid load, and if urine pH stays above 5.5 despite blood pH dropping to 7.3 or below, distal RTA is confirmed. Because this test frequently causes nausea and vomiting, a combined furosemide-fludrocortisone test has emerged as a gentler alternative with 100% sensitivity and specificity when using a urine pH cutoff of 5.3. Genetic testing can confirm hereditary forms and identify the specific gene involved.
Treatment and Long-Term Management
The cornerstone of RTA treatment is alkali therapy, replacing the bicarbonate the body is losing or failing to produce. The form and amount depend on the type.
For type 1 (distal) RTA, adults typically take sodium bicarbonate or sodium citrate in divided doses throughout the day. The amounts needed are relatively modest because the distal tubule’s job is smaller in scope. Children may need proportionally higher amounts relative to body weight, and the dose is adjusted as they grow. With consistent treatment, kidney stone formation slows, bone health improves, and children resume normal growth trajectories.
Type 2 (proximal) RTA requires significantly more alkali because any bicarbonate you replace gets filtered and promptly wasted by the defective proximal tubule. The goal is to keep blood bicarbonate near the normal range of 22 to 24 mEq/L, which can be difficult to sustain. Citrate salts are often better tolerated than straight sodium bicarbonate and carry the added benefit of raising urinary citrate. If Fanconi syndrome is present, phosphate and vitamin D supplements may also be necessary to protect bones.
Type 4 RTA often does not require bicarbonate supplementation at all. The priority is correcting hyperkalemia through dietary potassium restriction, adjusting medications that suppress aldosterone, or in some cases replacing the missing hormone. Treating the underlying cause, especially optimizing blood sugar control in diabetes, can improve the acidosis on its own.
Across all types, monitoring involves periodic blood tests to check bicarbonate, potassium, and kidney function, along with urine tests and imaging when kidney stones or nephrocalcinosis are a concern. For children, tracking growth velocity is an essential part of follow-up, since a plateau in growth can signal inadequate treatment before blood tests change.