Glomerular proteinuria describes the presence of an atypical amount of protein in the urine. It results from damage to the kidney’s filtering units, known as glomeruli. It is a sign of kidney damage, rather than a distinct disease itself.
The Kidney’s Filtering System
The kidneys filter waste products and excess water from the blood to produce urine. Each kidney contains approximately one million tiny filtering units called nephrons. Within each nephron, a glomerulus acts as a microscopic strainer.
Blood flows into the glomerulus. Healthy glomeruli allow small waste products and excess water to pass into the urine, while larger molecules like proteins are retained in the bloodstream. This selective filtration is achieved by a three-layered barrier, including specialized cells called podocytes with filtration slits, and a negatively charged basement membrane that repels negatively charged proteins like albumin.
Why Protein Leaks: Underlying Causes
Damage to the glomeruli can cause them to become “leaky,” allowing proteins that should remain in the blood to escape into the urine. Several conditions can lead to this damage.
Diabetes
High blood sugar levels, a hallmark of diabetes, damage the glomerular filters. This damage, known as diabetic nephropathy, is a leading cause of kidney disease globally. Elevated glucose levels increase pressure within the glomeruli, contributing to scarring and thickening of their walls. This impairs filtering capacity and leads to protein leakage.
High Blood Pressure (Hypertension)
Uncontrolled high blood pressure strains and injures small blood vessels throughout the body, including those within the glomeruli. Over time, this sustained pressure causes these vessels to narrow, weaken, or harden, reducing kidney filtration. This creates a challenging cycle, as kidney damage also makes blood pressure more difficult to manage.
Glomerulonephritis
Glomerulonephritis refers to a group of diseases characterized by inflammation and damage to the glomeruli. This inflammation can be caused by infections, such as post-streptococcal glomerulonephritis, or by autoimmune diseases where the body’s immune system attacks its own kidney cells. Examples include IgA nephropathy, where immune complexes deposit in the glomeruli, or lupus nephritis, a kidney inflammation linked to systemic lupus erythematosus.
Other Systemic Diseases
Various systemic conditions affect glomerular function. Amyloidosis, a disorder where abnormal proteins build up in organs, damages the kidneys’ filtering system. Certain infections, like HIV or hepatitis B and C, also lead to glomerular inflammation and progressive kidney damage.
Medications
Some medications contribute to kidney damage and subsequent proteinuria. Non-steroidal anti-inflammatory drugs (NSAIDs) impair blood flow to the kidneys, causing protein leakage. Certain antibiotics, chemotherapeutic agents, and some drugs for autoimmune or psychiatric disorders are associated with drug-induced kidney injury and proteinuria.
Identifying Protein in Urine
Detecting and measuring protein in urine involves several common diagnostic methods. A urine dipstick test serves as a quick initial screening tool. A paper stick dipped into a urine sample changes color if significant protein is present, offering a qualitative estimate.
For a more precise and quantitative assessment, healthcare providers often use the urine albumin-to-creatinine ratio (UACR) or protein-to-creatinine ratio (UPCR). These tests measure the amount of protein, particularly albumin, relative to creatinine in a single urine sample. Creatinine is a waste product consistently excreted in urine, so measuring protein against it helps account for variations in urine concentration.
Historically, a 24-hour urine collection was considered the most accurate method for measuring total protein excreted over a full day. This method, though less convenient, involves collecting all urine produced over a 24-hour period. While still used in specific situations, the UACR and UPCR are now more frequently employed due to their convenience and reliability.
What Glomerular Proteinuria Means
A diagnosis of glomerular proteinuria indicates underlying kidney damage. It signals an increased risk for the progression of chronic kidney disease (CKD), a gradual loss of kidney function that can eventually require dialysis or a kidney transplant. Proteinuria is also recognized as an independent risk factor for cardiovascular disease and other health complications.
Managing glomerular proteinuria primarily involves addressing its underlying cause. For individuals with diabetes, strict blood sugar control slows damage to the glomeruli. Similarly, for those with high blood pressure, effective blood pressure management helps reduce strain on kidney vessels.
Healthcare providers often prescribe kidney-protective medications, such as angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor blockers (ARBs). These medications work by reducing pressure within the glomeruli, which helps decrease protein leakage and protect kidney function. Lifestyle modifications also play a role, including dietary adjustments like reducing sodium intake, regular physical activity, and stopping smoking, all supporting kidney health and mitigating disease progression.