Doctor, why is my urine frothy (foamy)?
Proteins and other substances. All humans have a small amount of protein in the urine. This is normal (and probably) the cause. Like soap, proteins have certain properties that cause them to react in liquid and create bubbles or foam.
Urine is normally pale yellow to dark amber in color, and flat.
Foamy urine is characterised by the appearance and persistence of multiple layers of small to medium bubbles in your urine in the toilet bowl. This appearance of a single layer of larger bubbles on weeing, that quickly dissipate, is normal.
It usually happens more when your bladder is full and the urine is hitting the toilet fast enough to stir up the water – and/or if the urine is concentrated.
A urinary dipstick that is negative for protein will exclude any serious cause.
Even if it mildly positive, only about one third of patients with this complaint will be found to have abnormal proteinuria (protein in the urine). And when this is the case, the urinary protein levels are usually just above normal. In other words even then, there is no serious cause. Hence most cases of ‘foamy urine’ can be assumed to be normal.
What causes a liquid to be foamy?
Foam forms by trapping pockets of gas in liquid with the help of a surfactant. A surfactant is an organic compound (substance) that is amphiphilic (or amphipathic); meaning it contains both hydrophilic and hydrophobic ends.
Hydrophilic means ‘water loving’; hydrophobic means resistant to water. Hydrophilic molecules get absorbed or dissolved in water, while hydrophobic molecules only dissolve in oil-based substances.
A surfactant diffuses in water and is absorbed at the air-water interface, between where the water’s insoluble hydrophobic ends aggregate to form a bubble.
The cleaning property of soap also comes from its amphiphilic structure. When placed in system consisting of aqueous (water) and organic solvents (fatty dirt), soap will separate them.
Furthermore, the foam in beer (‘beer head’) is dependent on an amphipathic protein (lipid transport protein 1) derived from barley.
OK, so what causes urine to be foamy?
As we said above – proteins and other substances (probably).
In normal urine, proteins (or polypeptides) which have amphiphilic properties, can act as a surfactant and form foam in the urine. But it is not known exactly which proteins etc are primarily responsible for this issue.
Other factors
Also certain free amino acids share this property and can contribute to foam formation. The classic example of this is aminoaciduria that can be seen in Fanconi syndrome; where amino acids such as methionine and tyrosine (both of which have strong amphiphilic properties) can cause frothy urine.
Phospholipids, a constituent of the lipid bilayers of cell membranes, are also amphiphilic. Ruptured cells releasing membrane phospholipids in the urine (as in microscopic haematuria and/or pyuria without proteinuria), may also contribute to formation of urine foam.
Foamy urine and kidney disease
So it is not surprising that proteinuric (i.e. more than normal) kidney syndromes – such as acute or chronic glomerulonephritis, lupus nephritis and diabetic nephropathy – can present with foamy urine. If severe, this is called nephrotic syndrome.
Most of these diseases are ‘glomerular’ – i.e. cause symptoms by damaging the glomeruli (tiny filters) of the kidney. But, as we say, most people with foamy urine do not have significant kidney disease (glomerular or otherwise).
Foamy urine. (A) Fresh urine from a patient with nephrotic syndrome. (B) List of amphiphilic metabolites in normal human urine.
So should foamy urine be investigated?
If a urinary dipstick is negative (for protein), no further tests are required.
CKDEx also does not recommend investigating foamy urine in patients who lack significant proteinuria on urinary ACR (albumin-to-creatinine ratio) (10 mg/mmol or less).
Summary
We have answered the question ‘doctor, why is my urine frothy?’ It usually means nothing and should not be investigated. We hope this has been helpful.
Other resources
This is a good review article on the subject: Khitan, 2019
History of frothy (foamy) urine (human and medical interest in this subject goes back to the 6th century BC, and probably earlier)
Last Reviewed on 22 March 2024