Absent Kidney
An absent kidney refers to the finding of only one kidney where two are normally expected. This may be due to congenital absence (renal agenesis, in which one kidney never developed), an acquired cause such as prior surgical removal (nephrectomy) or severe injury, or a kidney that is present but so shrunken or nonfunctioning that it is not readily identified on imaging. Many people with a single functioning kidney live entirely normal lives, but the condition has important implications for long-term kidney health and requires periodic monitoring. Evaluation combines imaging (ultrasound, CT, or MRI) and kidney function testing.
What is it?
The kidneys are two bean-shaped organs located in the upper back part of the abdomen, one on each side of the spine. They filter waste and excess fluid from the blood to produce urine, help regulate blood pressure, balance electrolytes, activate vitamin D, and produce hormones that support red blood cell production and bone health. Most people are born with two kidneys, but a significant number of people have only one kidney—either because one never developed or because one was removed or destroyed after birth.
An absent kidney can result from several causes. Congenital unilateral renal agenesis is the condition of being born with only one kidney because the other never formed during fetal development. It occurs in roughly 1 in 1,000 to 1 in 2,000 live births and is more common in males. It may be identified incidentally on imaging at any age, during prenatal ultrasound, in evaluation of other congenital conditions, or during workup for kidney-related symptoms. Bilateral renal agenesis (absence of both kidneys) is a much rarer and severe condition that is typically diagnosed prenatally and is generally not compatible with life outside the womb without specialized intervention.
Acquired absence of a kidney refers to situations in which a kidney was originally present but is no longer functioning or physically present. Common causes include surgical removal (nephrectomy) for conditions such as kidney cancer, kidney donation for transplantation, severe kidney injury or trauma, or certain infections, developmental abnormalities, or vascular conditions that caused the kidney to shrink and stop functioning over time (an “atrophic” or “nonfunctioning” kidney).
An additional consideration is a kidney that is anatomically present but very small, shrunken (atrophic), or ectopic (located in an unusual position, such as the pelvis). Such a kidney may not be readily identified on initial imaging and may be mistaken for an absent kidney until more careful evaluation is performed.
Congenital unilateral renal agenesis is sometimes associated with other congenital conditions. In women, it may be associated with anomalies of the reproductive tract (Mullerian duct anomalies, such as absence of parts of the uterus, cervix, or vagina), including the Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome variant. In men, it may be associated with anomalies of the seminal vesicles or vas deferens. Various other congenital anomalies of the urinary, cardiovascular, gastrointestinal, or musculoskeletal systems can also occur. Genetic syndromes such as VACTERL association, branchio-oto-renal syndrome, and others may include renal agenesis as one component.
Most people with a single functioning kidney—whether congenital or acquired—have no symptoms specifically from having one kidney and live entirely normal lives. The remaining kidney typically undergoes compensatory hypertrophy, growing larger and filtering more effectively to help make up for the missing kidney. In many cases, overall kidney function remains within the normal range for many years.
Over time, however, some individuals with a single kidney may develop findings that reflect the increased workload on the remaining kidney or the effects of other conditions. Possible long-term findings can include high blood pressure, protein in the urine (proteinuria), and gradual reduction in kidney function (chronic kidney disease). The risk of these changes depends on many factors, including the age at which the second kidney was lost or absent, the health and structure of the remaining kidney, and the presence of other conditions such as diabetes, high blood pressure, obesity, and family history of kidney disease. Overall, most people with a single kidney maintain good kidney health throughout life with appropriate care and monitoring.
Diagnosis of an absent kidney is typically made through imaging. Kidney ultrasound is often the first-line test and can identify the presence, size, and location of the kidneys. CT provides detailed anatomical information and can identify small or ectopic kidneys, evaluate the ureters and bladder, and assess for associated anomalies. MRI can also provide detailed anatomical information without radiation. Nuclear medicine scans, such as MAG3 or DMSA renal scans, can assess kidney function when needed. Blood tests for creatinine and estimated glomerular filtration rate (eGFR), along with urine tests for protein and blood, evaluate kidney function. Blood pressure measurement is a routine part of assessment.
When congenital renal agenesis is identified—particularly in women—imaging of the reproductive tract (often with pelvic ultrasound or MRI) may be recommended to evaluate for associated Mullerian anomalies. In men, imaging or evaluation of the seminal vesicles and vas deferens may be considered when clinically indicated. Evaluation for other congenital anomalies is guided by the clinical context.
Genetic counseling and, in some cases, genetic testing may be appropriate when congenital renal agenesis is part of a broader syndrome, when there is a family history of kidney anomalies, or when reproductive planning is a consideration.
Important to Know
Management of an absent kidney focuses on protecting the remaining kidney over the long term, monitoring for early signs of kidney damage or dysfunction, and addressing associated conditions when present. Care is typically coordinated by primary care clinicians and, when needed, nephrologists (kidney specialists), urologists, and other specialists.
For most patients with an absent kidney and normal function of the remaining kidney, no specific treatment is required beyond good general health practices and periodic monitoring. Recommended measures include regular check-ups with blood pressure measurement, periodic blood tests (creatinine and eGFR), and periodic urine tests for protein and blood. The frequency of monitoring is individualized and typically ranges from annual to less frequent visits in stable patients without additional risk factors.
Blood pressure control is one of the most important aspects of long-term care. High blood pressure can damage the remaining kidney over time, and treatment is often more aggressive in patients with a single kidney than in the general population. Medications that both control blood pressure and provide direct kidney protection—such as ACE inhibitors or angiotensin receptor blockers—are often preferred, particularly when protein is present in the urine, though selection is individualized.
Avoidance of medications and substances that can harm the kidneys is important. Nonsteroidal anti-inflammatory drugs (NSAIDs, such as ibuprofen and naproxen) can affect kidney blood flow and function and should generally be avoided for regular or long-term use in people with a single kidney. Careful attention to medication dosing based on kidney function is important, and certain contrast agents used in imaging may require special consideration; these decisions are made in consultation with clinicians. Avoiding smoking and limiting alcohol also support kidney health.
Management of other health conditions that affect the kidneys is essential. Diabetes should be well controlled to reduce the risk of kidney damage. Cardiovascular risk factors, obesity, and other conditions should be addressed. A generally healthy diet, adequate hydration, and regular physical activity all support overall kidney and general health.
Contact and collision sports (such as football, rugby, and hockey) have historically raised concern about the risk of injury to the remaining kidney. Current guidelines from pediatric and sports medicine organizations recognize that the actual risk is relatively low, and decisions about participation are typically individualized based on the specific sport, the position of the remaining kidney (particularly in ectopic kidneys, which may be more vulnerable), the use of protective equipment, and shared decision-making between the patient, family, and healthcare team.
For patients with associated Mullerian anomalies or other congenital conditions, care is coordinated with gynecology, urology, and other relevant specialists. Reproductive planning, evaluation of fertility, and psychological support may all be important components of care.
For patients with an acquired absent kidney due to nephrectomy for kidney cancer, ongoing surveillance for cancer recurrence is important, as is monitoring of the remaining kidney. For kidney donors, care follows established protocols for donor follow-up, with attention to blood pressure, kidney function, and overall health.
Care during pregnancy in patients with a single kidney requires special attention. Most women with a single kidney and normal kidney function have uncomplicated pregnancies, but blood pressure, kidney function, and protein in the urine should be monitored more closely. Coordination with obstetrics and, when appropriate, maternal-fetal medicine and nephrology is important.
Care is typically coordinated by primary care clinicians and, when needed, nephrologists, urologists, and other specialists. Imaging and laboratory findings are interpreted alongside the patient’s symptoms, examination, family history, and broader clinical context rather than in isolation.
Patient education plays an important role. Understanding that most people with a single kidney live normal lives, the importance of long-term monitoring and protective measures, the rationale for medication choices and lifestyle recommendations, warning signs of kidney problems, and, when relevant, the implications for family members all contribute to better outcomes.
Red flag symptoms include severe back or flank pain (particularly on the side of the remaining kidney), significant decrease in urine output, blood in the urine, foamy or dark urine, severe swelling of the face or legs, severe hypertension, symptoms of severe electrolyte abnormalities (severe weakness, muscle cramps, irregular heartbeat), signs of urinary tract infection with fever, severe abdominal pain, or rapid clinical deterioration. These warrant prompt or urgent medical evaluation, as they may indicate acute injury or dysfunction of the remaining kidney, obstruction, infection, or other serious problems.