Your doctor will try to find out why your child has proteinuria, though this is not always possible. Proteinuria may be mild or last for a short time, or it may be more severe or last for a long time.

How proteinuria happens

The kidneys have many tiny filters (glomeruli). They normally help remove extra water and waste, which are passed out in urine, and make sure the body keeps what it needs, including protein.

In proteinuria, the glomeruli become more leaky than usual.

About protein

Protein is an important part of our diet and is in most foods. When we eat protein, it is digested (broken down) in the stomach and intestines and taken into the blood.

There are different types of proteins in the blood. The most common type is called albumin.

Often, only albumin is found in the urine – this is called albuminuria. If there is only a very small amount of albumin in the urine, this is called microalbuminuria.

Causes of mild or short-term proteinuria

Proteinuria may be mild if it happens during the daytime with only small amounts of protein leaked, or it lasts for a short time during a fever or urinary tract infection.

Orthostatic proteinuria

Sometimes the kidneys leak a small amount of protein during the daytime, especially when standing up and after exercise. This is called orthostatic proteinuria (‘orthostatic’ refers to standing up) or postural proteinuria. In these cases the proteinuria is harmless.

During a fever or urinary tract infection

The kidneys may also leak some protein during a fever or a urinary tract infection (UTI), when germs get into the urine and travel up the urinary system. The proteinuria usually disappears after the child gets better, and does not cause any long-term problems.

More about how the kidney works and protein leak

How the kidney works

Inside each kidney, there are about one million nephrons. Each nephron is made up of a glomerulus (when we talk about more than one glomerulus, we say glomeruli), and a renal tubule.

  • Each glomerulus acts like a sieve, helping to remove extra water and waste from the body, and holding on to blood cells and protein, which the body needs.
  • Blood flows into the kidneys and to each glomerulus.
  • Most of the water and some other substances in the blood pass through the glomeruli.
  • This liquid flows into the renal tubule. Most of this liquid moves back into the bloodstream. The rest of it becomes urine.
  • The urine leaves the kidney by the ureters and goes into the bladder, where it is stored until we are ready to go to the toilet.

Protein leak in the glomeruli

Most proteins are too big to pass through the glomeruli, and stay in the bloodstream. In proteinuria, the glomeruli become more leaky than usual. More proteins than usual pass through the glomeruli, and leave the body in urine.

Protein leak in the renal tubules

Some small proteins do get through healthy glomeruli and pass into the renal tubule. Most of these are re-absorbed into the bloodstream.

If the tubules are damaged, they may be unable to remove the proteins back into the bloodstream. The proteins are then lost into urine.

Causes of severe or persistent proteinuria

A small number of children have large amounts of protein in their urine, or have persistent proteinuria (that does not go away). These may be signs of damage in the kidneys.

Nephrotic syndrome

Nephrotic syndrome happens when a lot of protein is leaked into the urine. This means that there is less protein in the blood. This amount of proteinuria is called nephroticrange proteinuria.

Children with nephrotic syndrome have swelling around their eyes or their legs or feet. Many children with this condition get better after taking medicine for a few weeks, but a few need special treatment.

This type of proteinuria is very rare. It is seen in 1 or 2 children in every 100,000.


Some children are born with vesicoureteral reflux (VUR). In VUR, some urine refluxes (goes back up) towards the kidneys.

This makes it easier for germs that cause UTI to travel all the way up to the kidneys, where they cause a more serious infection and illness. An infection in the kidney is called pyelonephritis. When this happens, the kidneys can get scars and leak protein.

Renal dysplasia

Some children who have reflux also have renal dysplasia. One or both kidneys did not develop normally while they were growing in the womb. The affected kidney may leak protein.

Kidney conditions

Some kidney conditions that may cause proteinuria include:

  • glomerulonephritis
  • acute kidney injury
  • vasculitis
  • diabetes, when it affects the kidneys
  • diseases that affect the renal tubules (the kidney’s tubes that carry the liquid that will become urine) – these are very rare, and include Dent disease, Lowe syndrome, Fanconi syndrome and aminoaciduria.


Hypertension is blood pressure that is too high. Occasionally, this can be linked to proteinuria.

More about types of proteins and kidney conditions


There is more albumin than any other type of protein in our blood. Most of the time, the protein that is lost in the urine in proteinuria is albumin. This is because albumin is smaller than other proteins. If a glomerulus (kidney filter) is only slightly damaged, it can let albumin pass through.


Globulin is another type of protein. This protein is larger than albumin. If a glomerulus is more seriously damaged, it can let globulins pass through.

Other proteins

Other types of protein are smaller and so can easily pass through the glomeruli and into the renal tubules. The renal tubules normally remove these proteins back into the bloodstream. If the renal tubules are not working properly, they may not remove the proteins,

which means they are lost in the urine. This happens in:

  • rare diseases of the renal tubules – including Dent disease, Lowe syndrome, Fanconi syndrome and aminoaciduria
  • acute kidney injury – the kidneys suddenly stop working.
  • acute interstitial nephritis – inflammation of the kidneys, sometimes caused by the use of some medicines.

Kidney conditions

  • Glomerulonephritis is a group of conditions that cause inflammation in the glomeruli. Children with this condition have blood and protein in their urine, and usually have swelling in their body, especially around their face and legs. Some children need special treatment.
  • Acute kidney injury means that the kidneys stop working as well as they should – this happens over a short period of time. Many children get better after a few weeks without treatment, but others need to take medicines or have more intensive treatment.
  • Vasculitis happens when the blood vessels in different parts of the body become inflamed – this can sometimes affect the blood vessels in the kidneys.
  • Children who have diabetes are not able to convert (change) sugar into energy in the way they should. This can sometimes cause problems in their kidneys – called diabetic nephropathy – although this is rare in children. It usually happens after years of living with diabetes, and is more likely if the diabetes is not well controlled.

More information

  • Urinary tract infection (UTI)

    Urinary tract infections happen when germs get into the urine (wee) and travel into the urinary tract.

  • Acute kidney injury

    Acute kidney injury (AKI) is when the kidneys stop working over a short period of time (a few days or a few weeks).

  • Glomerulonephritis

    A group of conditions in which the kidneys are inflamed and leak blood and protein into the urine.