What causes protein in urine when you have lupus?

In lupus, the immune system mistakenly attacks the body's own tissues and organs, including the kidneys. This inflammation can damage the small blood vessels in the kidneys, known as glomeruli, which are responsible for filtering waste products from the blood. When the glomeruli are damaged, they can allow protein to leak from the blood into the urine, a condition called proteinuria.

Several factors can contribute to proteinuria in lupus:

Immune complex deposition: In lupus, immune complexes, which are composed of antibodies and antigens, can deposit in the glomeruli. These immune complexes activate the complement system, a part of the immune response, which leads to inflammation and damage to the glomeruli. This damage can result in proteinuria.

Anti-glomerular basement membrane (anti-GBM) antibodies: In some cases of lupus, the immune system produces antibodies that specifically target the glomerular basement membrane, the layer that separates the glomerular capillaries from the Bowman's capsule. These antibodies, known as anti-GBM antibodies, can directly damage the glomeruli and cause proteinuria.

Lupus nephritis: Proteinuria is a common feature of lupus nephritis, which is a type of kidney disease caused by lupus. Lupus nephritis occurs when the inflammation and damage caused by lupus affect the kidneys. The severity of proteinuria can vary depending on the stage and activity of lupus nephritis.

Other factors: Certain medications used to treat lupus, such as non-steroidal anti-inflammatory drugs (NSAIDs) and some chemotherapy drugs, can also contribute to proteinuria.

It's important to note that proteinuria is not always a sign of kidney damage in lupus. In some cases, it can be a temporary finding that resolves over time. However, persistent or significant proteinuria can indicate kidney involvement and requires close monitoring and management by a healthcare professional to prevent or slow the progression of kidney disease.