Urea

Urea is formed in the liver during the breakdown of proteins from food and from the body itself. The kidneys filter urea from the blood and excrete it in the urine. If the kidneys work less well or receive less blood flow, urea in the blood rises. How much urea there is also depends on how much protein you eat and how much fluid you take in. Urea is therefore almost always assessed together with creatinine.

• Reduced kidney function, together with creatinine and eGFR
• Reduced blood flow to the kidneys, for example with dehydration or heart failure
• Increased breakdown of protein, for example after a protein-rich meal, with fever or with bleeding in the stomach or intestine

With an abnormal urea, the GP first looks at creatinine and eGFR and at drinking, diet and medicines. If only urea is slightly raised, a repeat measurement after drinking well usually follows. If creatinine or eGFR is also abnormal, the approach of the Dutch GP guideline (NHG) on chronic kidney damage follows, with a repeat measurement and a urine test for albumin.

Urea is a less reliable measure of kidney function than creatinine, because the value depends strongly on eating, drinking and protein breakdown. Kidney function is therefore assessed with the eGFR, which is calculated from creatinine. Urea mainly has meaning alongside creatinine: if urea rises much more than creatinine, this points more to dehydration or increased protein breakdown than to kidney damage.

• Protein-rich food, protein supplements and heavy exertion raise urea without the kidneys working any worse.
• Drinking too little before the blood draw raises the value; drinking a lot lowers it.
• Corticosteroids, fever and bleeding in the stomach or intestine raise urea.
• Urea only rises clearly once kidney function has already declined considerably; a normal value does not rule out kidney damage.

There is no Dutch figure for how often an abnormal urea occurs. According to the Dutch GP guideline (NHG), chronic kidney damage, in which urea eventually rises, occurs in about 1 in 10 adults and more often at older age. A slightly raised urea due to drinking little or protein-rich food is much more common than kidney damage.

Mainly useful as an addition to creatinine and eGFR, for example with diabetes, high blood pressure, cardiovascular disease, dehydration or medicines that put strain on the kidneys. In strength athletes and with the use of testosterone or anabolic substances, urea is often measured together with creatinine; a protein-rich diet then makes the value harder to interpret. In healthy adults without symptoms or risk factors, the chance of reduced kidney function is small, and urea alone adds little to creatinine and eGFR.

• A slightly raised urea due to protein-rich food or drinking too little can cause unfounded worry about the kidneys.

• NHG-Standaard Chronische nierschade (M109)
• NVKC, allesovertesten.nl: Ureum
• NVKC, allesovertesten.nl: Kreatinine
• Thuisarts.nl: Ik heb nierschade
• Thuisarts.nl: Uitdrogen