Albumin Testing In Nephrology

Understanding the significance of albumin in physiology

What is albumin?

Human serum albumin (HSA) is one of the most abundant proteins in our blood. It has multiple roles in physiology and medicine:

Oncolytic pressure regulation
A primary antioxidant to protect against free radical damage
Transport of fatty acids, hormones, and other small molecules
Transport of medicines via the circulatory system
Biomarker of kidney disease

Albumin is made in the liver and immediately secreted into the bloodstream, where it travels around for a LONG time. It is one of the most stable proteins, with a half-life of 19 days. During these travels, albumin leaves the bloodstream to interstitial spaces where it is returned to the circulatory system via lymph. Around 10 – 15 g of protein is secreted into the bloodstream each day, making it the most abundant protein in plasma (3.4 to 5.4 g/dL) (Belinskaia et al., 2021; Peters, 1995).

Why make so much albumin?

The amount of albumin makes the blood plasma colloid-like, which establishes the osmotic pressure across the vascular endothelium. Low albumin levels in plasma result in fluid imbalances characterized by edema.

As an antioxidant, albumin can bind metal ions, fatty acids, and nitric oxide (NO) to prevent them from creating reactive oxygen species (ROSs). Copper and iron are potent generators of ROSs when they interact with hydrogen peroxide (H₂O₂). If bound to albumin, free Cu(II) and Fe(II) are much less likely to encounter H₂O₂, and therefore, reduces ROS production. Similarly, fatty acids and NO bound to albumin are protected from producing ROSs (Roche et al., 2008). Between its ability to bind a variety of potential free radical producing molecules and its overall abundance, albumin may be one of the body’s most important antioxidants. But there is more reasons why albumin is important…

Human albumin structures bound to different drugs — Human serum albumin transports hormones, fatty acids and many drugs. From upper left to bottom right: azaprazone (anti-inflammatory), camptothecin (oncologic drug), propofol (anesthetic), halothane (anesthetic), octadecanoic acid, prostaglandin, bilirubin and ibuprofen. Source: PDB entries 1e7a, 1e7b, 1e7i, 2bx8, 2vue, 3a73, 4i9k, 6oci.

Albumin carries medicines

For drug development pipelines, albumin plays a critical role in transporting various exogenous substances (Catalano et al., 2024).

Albumin carries drugs including propranolol, warfarin, and methotrexate to name a few. Drug absorption, distribution, metabolism, and excretion (ADME) are influenced by the affinity of the drug to albumin. Pharmacodynamic and pharmacokinetic effects are influenced by the drug-albumin affinity, sometimes in a positive way. In fact, albumin is used as a carrier to facilitate distribution of a medicine in patients (Liu et al., 2017). Unfortunately, if the drug binds albumin too tightly, the association can make the drug ineffective.

For medicine, determining the amount of albumin in the body has been an important biomarker or symptom that the liver is functioning properly. But albumin is an important biomarker of kidney disease also.

Is albumin found in urine?

When kidneys function within a normal range, albumin that enters the Bowman’s capsule can cross the glomerular basement membrane but typically doesn’t cross if bound to other substances. The glomerular filtration of albumin is highly variable depending on strains of model organisms and nutritional status (Sandovalet al., 2012).

The structure of a nephron with blood, glomerulus, bowman’s capsule, proximal tubule, and collecting duct.

Interestingly, albumin entering the proximal tubules is reabsorbed by cellular surface receptors, including megalin, cubulin, and the neonatal Fc receptor. When albumin binds to the receptor, it is internalized by clathrin-coated invaginations and degraded into amino acids by the lysosome. Alternatively, proximal tubule cells can transcytose the albumin back to the body (Dickson et al., 2014; Molitoris et al., 2022).

Why does albumin in urine indicate kidney dysfunction?

When kidneys start releasing albumin into the urine, it can indicate illnesses such as nephrotic syndrome, chronic kidney disease, or end-stage renal disease. Called albuminuria, the excretion of albumin in urine disrupts the osmotic balance in the body and results in edema.

Microalbuminuria or “moderately increased albuminuria” is when a small amount of albumin is found in the urine, which can indicate high blood pressure, type I or II diabetes, metabolic syndrome, or kidney disease. Early detection of high albumin and treatment for the underlying cause will reduce the risk for further kidney damage, serious cardiac events, or stroke.

Albumin ELISA Kits

Research into the root causes for microalbuminuria and albuminuria are essential to develop new treatments, identify new biomarkers, and find the root genetic or metabolic causes for kidney function.

Why choose Exocell Albumin ELISAs?

Exocell Albumin ELISAs assess kidney function through ELISA-based microalbuminuria measurement from human, mouse, and rat samples. Part of our Exocell ELISA portfolio, each Albumin ELISA kit provides quantitative detection and monitoring of albumin concentrations in plasma or urine. These Albumin ELISA assays are specific to albumin, easy to use, and available to assist research efforts in nephrology. We also offer a creatinine assay to determine the albumin-to-creatinine ratio (ACR).