Atypical Hemolytic Uremic Syndrome (aHUS)

Diseases and conditions Rare kidney diseases and disorders Hemolytic uremic syndrome

Last Updated: August 26, 2025

Medically reviewed by NKF Patient Education Team

Atypical uremic hemolytic syndrome is a rare disease that can cause kidney failure and is life- threatening. Early diagnosis and treatment are key.

Table of Contents

About aHUS

Atypical hemolytic uremic syndrome (aHUS) is a rare and complex disease that often causes kidney failure and is life-threatening. In aHUS, a part of the immune system called the complement system becomes overactive, which leads to blood clots. These clots prevent blood flow and damage various organs, especially the kidneys.  Early diagnosis and treatment can help prevent long-term kidney damage and other complications. aHUS may become chronic, which means having repeated episodes of the disease.

Signs and Symptoms

Common signs and symptoms of aHUS include:

  • Fatigue (feeling very tired) and weakness
  • Confusion, seizures, or stroke-like symptoms (vision or speech problems or unable to move parts of body)
  • Shortness of breath
  • Decreased urine output (peeing less or not at all)
  • Bleeding or bruising
  • Swelling in legs and face
  • High blood pressure
  • Sudden and severe abdominal (belly) pain
  • Loss of appetite
  • Flu-like symptoms such as joint and muscle pain

aHUS does NOT usually start out with severe symptoms

Causes

Causes of aHUS include: 

  • Genetic mutations (changes in the genes) passed along in families that can cause diseases
  • Genetic mutations that happen on their own with no link to family history (spontaneous mutations)
  • Proteins called autoantibodies that make the body attack itself
  • Those that are unknown (idiopathic)  

Triggers of aHUS should not be confused with the causes of aHUS mentioned above. We all get colds and infections, and the body’s immune system can fight them off. But in aHUS, a common cold can trigger an aHUS episode because the body’s immune system is not reacting as it should.  Aside from infection, pregnancy is an important trigger for aHUS.

Complications

Complications of aHUS include 

  • Red blood cells breaking down (hemolytic anemia). Red blood cells carry oxygen throughout the body.
  • Platelets dropping very low (thrombocytopenia). Platelets clot the blood to prevent bleeding.
  • Waste products building up in the blood when the kidneys can’t make urine (acute kidney injury or acute kidney failure)
  • If left untreated, aHUS can lead to severe complications like kidney failure, heart and neurologic problems, and even death

Platelets clot the blood to prevent bleeding. In aHUS, platelet levels drop, but blood clots still happen because of problems with the complement system.]

After the life-threatening initial episode, people with aHUS are at risk of developing chronic (long-term) serious complications such as high blood pressure and kidney failure. 

Diagnosis

Making the diagnosis of aHUS is a complex process of “ruling out” all other causes of a disease.  This requires medical teams to eliminate conditions with similar signs and symptoms until aHUS is the only remaining option. Medical teams consider a combination of signs and symptoms, laboratory tests, and genetic tests to confirm a diagnosis. Genetic testing is especially crucial, as aHUS is often related to genetic mutations affecting the complement system. 

Tests

  1. Clinical evaluation includes medical history, symptoms, and a doctor’s physical exam to look for signs of aHUS.
  2. Blood tests are crucial in the diagnosis of aHUS and may include:
    • Complete blood count (CBC) to check for anemia and abnormal blood cell counts
    • Blood chemistry tests to evaluate kidney function and electrolyte levels
    • Coagulation tests to assess blood clotting function
  3. Kidney function tests that include serum creatinine and estimated glomerular filtration rate (eGFR) to evaluate kidney function.
  4. Urinalysis (urine tests) to detect abnormalities such as protein or blood in the urine, which may be signs of kidney damage.
  5. Genetic testing  for genetic mutations that affect the complement system.
  6. Imaging studies  like ultrasound, CT scans, or MRI may be used to assess the kidneys and other organs for signs of damage or abnormalities.
  7. Kidney biopsy may be done in some cases to examine a small sample of kidney tissue for signs of aHUS-related damage.
  8. ADAMTS13 testing for levels of a protein in the blood called ADAMTS13.  The level of this protein can clarify if a person has aHUS or thrombotic thrombocytopenia purpura (TTP).  TTP shares some signs and symptoms with aHUS but has a different underlying cause. 

Treatment

While aHUS is a severe and life-threatening disease, there are treatment options. Eculizumab blocks the complement system’s overactivity to prevent blood clots and is given as an infusion through a vein. A newer form of this drug is ravulizumab, which lasts longer and can be given less often. Treatment may be life-long to prevent repeat episodes of the disease.  Stopping or continuing treatment after an initial episode is based on each person’s specific medical needs.

For those with severe kidney damage, a kidney transplant can restore kidney function. However, it’s essential for transplant recipients to continue treatment to lower the risk of aHUS returning in the transplanted kidney.

A person’s symptoms may change over time. Frequent lab tests can detect another episode before the onset of symptoms, so that rapid treatment can be given to protect kidney function during a relapse. Don’t wait for severe symptoms, as often the only warnings will be some fatigue, bruising, or vomiting. Ask for lab tests at the first sign of any health or behavioral issues. 

Questions to ask

  1. What is genetic counseling?  In what ways can it help with both the diagnosis and treatment of aHUS?
  2. Where can I find a genetic counselor?
  3. Which types of doctors are part of the aHUS team?
  4. Where can I find support for managing my aHUS?
  5. Can I breastfeed if I’m receiving eculizumab or ravulizumab?
  6. Can I be a kidney transplant donor if I carry a genetic mutation for aHUS? Can I be a kidney transplant donor if aHUS runs in my family and my genetic test results are negative for aHUS?

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This content is provided for informational use only and is not intended as medical advice or as a substitute for the medical advice of a healthcare professional.
© 2025 National Kidney Foundation, Inc.