Urinary protein biomarker detection is used to determine the source of proteinuria, the degree of filtration barrier injury, and whether tubular injury is also present. Albumin, transferrin, IgG, and α2-macroglobulin differ in molecular size and in the conditions under which they enter urine. Combined detection can help distinguish early glomerular permeability changes, selective proteinuria, nonselective proteinuria, and post-renal interference.

Keywords: urinary protein biomarkers; urinary albumin; urinary transferrin; urinary IgG; urinary α2-macroglobulin; proteinuria classification; glomerular injury; tubular injury

1 Selection Logic of Urinary Protein Biomarkers

1.1 Sources of proteinuria

(1) Glomerular proteinuria

The glomerular filtration barrier consists of endothelial cells, the glomerular basement membrane, and the podocyte slit diaphragm. When charge selectivity or size selectivity of the barrier is impaired, plasma proteins can enter the urine. Increased albumin and transferrin usually suggest early or relatively selective changes in glomerular permeability, whereas increased IgG and α2-macroglobulin indicate leakage of larger molecular proteins.

(2) Tubular proteinuria

The proximal tubule is responsible for reabsorbing low-molecular-weight proteins. When tubular injury occurs, low-molecular-weight proteins such as β2-microglobulin and α1-microglobulin may increase. Albumin, transferrin, IgG, and α2-macroglobulin are mainly used for analyzing glomerular sources of proteinuria, but in drug-induced kidney injury, acute kidney injury, or mixed injury, tubular markers should be combined for interpretation.

(3) Post-renal proteinuria

Urinary tract bleeding, inflammation, stones, tumors, or sample contamination can introduce plasma proteins and large molecular proteins into urine. α2-Macroglobulin has an extremely large molecular weight and is normally difficult to filter through an intact glomerular barrier. If urinary α2-macroglobulin is elevated, urinary occult blood, red blood cells, white blood cells, and sampling contamination should be checked simultaneously.

1.2 Rapid selection pathway

2 Albumin Detection

2.1 Detection significance

(1) Early glomerular permeability changes

Albumin has a molecular weight of approximately 66 kDa and is the most commonly used glomerular injury marker in urinary protein detection. Increased urinary albumin usually indicates altered charge selectivity or permeability of the glomerular filtration barrier. It is suitable for diabetic nephropathy, hypertensive kidney injury, chronic kidney disease risk assessment, and screening of kidney injury in animal models.

(2) Screening for low-level proteinuria

Urinary microalbumin is more suitable for detecting low-level albuminuria. Routine urine protein dipsticks are not sensitive to low-level albuminuria. In studies of early glomerular barrier abnormalities, urinary microalbumin ELISA or immunological methods should be prioritized.

(3) Difference from total urinary protein

Total urinary protein reflects the total amount of all proteins in urine and cannot distinguish albumin, large molecular IgG, or low-molecular-weight tubular proteins. Albumin is more oriented toward screening for glomerular-source proteinuria, but albumin alone cannot determine proteinuria selectivity.

2.2 Detection methods and principles

(1) ELISA

Urinary albumin ELISA usually uses a double-antibody sandwich format. The capture antibody is immobilized in the reaction well. Albumin in the sample binds to the capture antibody, and then an enzyme-labeled detection antibody is added to form an immune complex. Substrate reaction generates a colorimetric or fluorescent signal. The signal intensity is related to albumin concentration and is quantified using a standard curve.

ELISA has high sensitivity and is suitable for urinary microalbumin, animal model urine samples, small-volume urine samples, and low-abundance sample detection. Samples from different species should be tested with species-matched kits to avoid falsely low values caused by insufficient antibody cross-reactivity.

(2) Immunoturbidimetry and immunonephelometry

Immunoturbidimetry and immunonephelometry rely on the formation of immune complexes between albumin and specific antibodies. These complexes increase turbidity or scattered light, and the instrument quantifies albumin based on changes in optical signals. These methods have a high degree of automation and are suitable for clinical batch testing and albumin/creatinine ratio calculation.

(3) Bromocresol green and bromocresol purple colorimetric methods

Bromocresol green and bromocresol purple are dye-binding methods. Under specific pH conditions, albumin binds to the dye, causing a change in the absorption spectrum, which can be used for rapid albumin quantification. These methods are simple and suitable for method comparison and batch screening, but they are sensitive to low urinary albumin concentrations, non-albumin interference, and urine matrix background.

2.3 Sample handling and interpretation

(1) Random urine correction

Albumin concentration in random urine is affected by water intake and urine concentration. Results should preferentially be expressed as the urinary albumin/creatinine ratio rather than comparing concentration values alone.

(2) Animal model samples

Mouse and rat urine volumes are limited, making high-sensitivity ELISA suitable. During collection, contamination from feces, feed powder, and blood should be minimized. Urinary creatinine correction should be performed when necessary.

(3) Result interpretation

Mild albumin elevation suggests early glomerular barrier abnormality. Marked albumin elevation without increased IgG may indicate selective proteinuria. Simultaneous elevation of albumin, transferrin, and IgG suggests an increased likelihood of nonselective proteinuria.

3 Transferrin Detection

3.1 Detection significance

(1) Early glomerular barrier abnormality

Transferrin has a molecular weight of approximately 76–80 kDa, slightly higher than albumin. Because its molecular weight and charge characteristics differ from those of albumin, urinary transferrin can serve as a complementary marker for observing early glomerular filtration barrier abnormalities.

(2) Analysis of selective proteinuria

In relatively selective proteinuria, medium-sized proteins are more likely to leak, while large proteins such as IgG leak less. If albumin and transferrin are elevated but IgG is not obvious, proteinuria usually still retains a certain degree of selectivity.

(3) Diabetic kidney injury research

Urinary albumin and urinary transferrin may increase in early diabetic kidney injury. If urinary albumin is near the borderline range, transferrin can add another dimension for observing glomerular barrier abnormalities.

3.2 Detection methods and principles

(1) ELISA

Urinary transferrin ELISA usually uses a sandwich immunoassay principle. A transferrin-specific capture antibody is immobilized in the reaction well. Transferrin in the sample is captured and then recognized by an enzyme-labeled detection antibody targeting another epitope. Quantification is performed through the enzymatic colorimetric signal.

This method is suitable for urine, serum, tissue extracts, and animal model samples. For urine testing, attention should be paid to the sample dilution ratio and standard curve range to avoid high-concentration samples exceeding the linear range.

(2) Immunoturbidimetry and immunonephelometry

Immunoturbidimetry or immunonephelometry quantifies transferrin based on optical changes after transferrin forms complexes with anti-transferrin antibodies. These methods are suitable for scenarios with large sample numbers and standardized detection platforms.

(3) Transferrin standards and controls

Transferrin standards can be used for standard curves, spike recovery, and method validation. Holo-transferrin, apo-transferrin, and partially iron-saturated transferrin are more suitable for method studies related to iron-binding status. If used only for urinary transferrin quantification, consistency of immunological recognition and the source of the standard are usually the key concerns.

3.3 Sample handling and interpretation

(1) Hematuria interference

Transferrin is a plasma-derived protein. In hematuria, urinary tract inflammation, or samples contaminated with blood components, urinary transferrin can increase passively and should not be directly interpreted as a glomerular filtration barrier abnormality.

(2) Combined interpretation with albumin

Simultaneous elevation of albumin and transferrin generally supports glomerular-source protein leakage. If transferrin is increased while albumin changes only slightly, sample quality, detection linear range, and urinary sediment results should be reviewed.

(3) Combined interpretation with IgG

Elevated transferrin without obvious IgG tends to indicate selective proteinuria. Elevation of both transferrin and IgG suggests broader barrier injury and reduced proteinuria selectivity.

4 IgG Detection

4.1 Detection significance

(1) Leakage of large molecular proteins

IgG has a molecular weight of approximately 150 kDa and is normally present at low levels in urine. Increased urinary IgG suggests impaired size selectivity of the glomerular barrier and entry of large molecular proteins into urine. It is an important indicator of nonselective proteinuria.

(2) Assessment of barrier injury severity

Compared with isolated albuminuria, increased IgG usually indicates more severe structural injury of the filtration barrier. Increased urinary IgG may occur in nephrotic syndrome, active glomerulonephritis, membranoproliferative lesions, or other severe glomerular diseases.

(3) Dynamic monitoring

During treatment, if albumin decreases but IgG remains persistently elevated, large molecular protein leakage may not have improved sufficiently. Urinary sediment, renal function, total urinary protein, and pathological background should be integrated for interpretation.

4.2 Detection methods and principles

(1) ELISA

Urinary IgG is commonly detected by sandwich ELISA. Anti-IgG capture antibodies bind IgG in urine, and enzyme-labeled detection antibodies recognize the IgG molecule and generate a colorimetric signal. Concentration is calculated using a standard curve. ELISA is suitable for low-abundance urinary IgG, animal model samples, and research samples.

(2) Immunoturbidimetry and immunonephelometry

Anti-IgG antibodies form antigen-antibody complexes with sample IgG, causing increased turbidity or scattered light. The instrument quantifies IgG based on signal changes. This method is suitable for batch detection with large sample numbers, but low-concentration urinary IgG, animal samples, and special matrix samples require methodological validation.

(3) IgG standards

Standards such as Human IgG, Mouse IgG, and Rat IgG can be used for standard curves, spike recovery, and batch-to-batch quality control. In animal models, species-matched standards and kits should be used to avoid species mismatch affecting the detected value.

4.3 Sample handling and interpretation

(1) Exclusion of hematuria

Hematuria can introduce plasma IgG and cause false elevation of urinary IgG. If urinary occult blood or red blood cells are markedly increased, IgG elevation should not be directly interpreted as glomerular leakage of large molecular proteins.

(2) Determining nonselective proteinuria

Simultaneous elevation of albumin, transferrin, and IgG usually suggests nonselective proteinuria. If IgG is markedly elevated and accompanied by decreased eGFR, abnormal urinary sediment, or casts, more active or more extensive glomerular injury should be considered.

(3) Combined interpretation with α2-macroglobulin

Increased IgG without increased α2-macroglobulin suggests large molecular protein leakage without obvious leakage of ultra-large proteins. Simultaneous elevation of IgG and α2-macroglobulin requires further differentiation between severe glomerular barrier disruption and post-renal protein sources.

5 α2-Macroglobulin Detection

5.1 Detection significance

(1) Abnormal appearance of ultra-large molecular proteins

α2-Macroglobulin has a molecular weight of approximately 720 kDa and is an ultra-large plasma protein. An intact glomerular filtration barrier usually does not allow it to pass. Therefore, the presence of α2-macroglobulin in urine has strong abnormal significance.

(2) Assessment of severe barrier disruption

If α2-macroglobulin and IgG are both elevated, this may suggest severe glomerular barrier disruption and a high degree of nonselective proteinuria. This marker is more suitable for severe barrier disruption assessment, post-renal source investigation, and large molecular proteinuria stratification.

(3) Evaluation of post-renal sources

α2-Macroglobulin can also increase due to urinary tract bleeding, inflammatory exudation, or blood contamination of samples. Because of its extremely large molecular size, result interpretation must be combined with urinary occult blood, urinary red blood cells, white blood cells, urinary tract infection background, and repeated sampling results.

5.2 Detection methods and principles

(1) ELISA

The basal level of α2-macroglobulin in urine is usually low, so high-sensitivity sandwich ELISA is more suitable. A solid-phase antibody captures α2-macroglobulin in the sample, and the detection antibody recognizes a different epitope. Quantification is performed through enzymatic color development.

During detection, special attention should be paid to linearity in the low-concentration range, sample dilution ratio, standard source, and batch-to-batch consistency. α2-Macroglobulin levels in urine are low; if the sample is over-diluted, the concentration may fall below the detection limit.

(2) Antibody validation

α2-Macroglobulin antibodies can be used for Western blot, immune assay development, or tissue localization analysis. For urine samples, because the protein has a large molecular weight and low abundance, sample pretreatment should minimize protein degradation, precipitation, and loss due to tube-wall adsorption.

(3) Standards and quality controls

α2-Macroglobulin standards can be used for standard curves, spike recovery, and method validation. Because this marker has a low basal level in urine, low-concentration quality control points are recommended to confirm reliability in the low-abundance range.

5.3 Sample handling and interpretation

(1) Combined interpretation with hematuria

If α2-macroglobulin elevation is accompanied by increased urinary occult blood, red blood cells, or white blood cells, post-renal origin or sample contamination should be prioritized for evaluation. In this situation, a single result should not be directly used to determine severe glomerular injury.

(2) Combined interpretation with IgG

Increased IgG without increased α2-macroglobulin suggests leakage of large molecular proteins but not obvious leakage of ultra-large proteins. Elevation of both IgG and α2-macroglobulin suggests severe nonselective leakage or a post-renal protein source.

(3) Value of repeat sampling

When α2-macroglobulin results are abnormal, clean-catch midstream urine or morning urine should be recollected, and urinary sediment should be tested simultaneously. If α2-macroglobulin decreases or becomes negative after repeat sampling, contamination or transient hematuria interference in the previous sample should be considered.

Table 1 Comparison of Four Types of Urinary Protein Biomarkers

6 Combined Detection and Result Classification

6.1 Selective proteinuria

Albumin and transferrin are markedly elevated, IgG is not elevated or only mildly elevated, and α2-macroglobulin is usually not elevated. This pattern suggests predominant leakage of medium-sized proteins, while the glomerular barrier still retains some ability to restrict large molecules. Selective proteinuria is often used to assess early barrier permeability changes, but it cannot replace etiological diagnosis.

6.2 Nonselective proteinuria

Albumin, transferrin, and IgG are all elevated, suggesting decreased size selectivity of the glomerular barrier. If IgG is markedly elevated and accompanied by abnormal urinary sediment, casts, or decreased eGFR, more extensive or more active glomerular injury should be considered. If α2-macroglobulin is also elevated, severe barrier disruption and post-renal sources should be further evaluated.

6.3 Mixed proteinuria

Albumin or IgG elevation accompanied by increased tubular markers such as β2-microglobulin and α1-microglobulin suggests coexistence of glomerular leakage and proximal tubular reabsorption impairment. Mixed patterns are common in drug-induced kidney injury, ischemia-reperfusion injury, advanced diabetic nephropathy, and inflammatory kidney injury.

7 Sample Handling, Method Boundaries, and Failure Causes

7.1 Sample and method boundaries

Random urine is suitable for clinical screening and dynamic observation in animal models, but urinary creatinine correction should be performed. Morning urine is suitable for low-level albuminuria and small-volume animal urine analysis. Twenty-four-hour urine is suitable for evaluating total excretion, but requires complete collection. Hematuric samples can indicate interference risk, but elevated IgG or α2-macroglobulin in such samples should not be directly interpreted as glomerular leakage.

Animal urine samples should be tested using species-matched ELISA kits. Mouse, rat, rabbit, and other samples should not be assumed to be compatible with human kits unless cross-reactivity and sample suitability have been clearly validated. Cell or tissue extracts can be used for methodological validation, but they cannot be directly equated with urinary proteinuria classification results.

7.2 Key control points

(1) Urine dilution

Urinary protein concentration is strongly affected by water intake, urine volume, and sampling time. For random urine, urinary protein/creatinine ratios are recommended, such as albumin/creatinine, IgG/creatinine, or α2-MG/creatinine.

(2) Hematuria and inflammation

IgG, transferrin, and α2-macroglobulin can all increase due to contamination with plasma components. When occult blood, red blood cells, or white blood cells are elevated, urinary sediment, red blood cell morphology, and repeat sampling should be used for interpretation.

(3) Linear range

A dilution-gradient pilot test should be performed before ELISA detection to ensure that samples fall within the standard curve linear range. Highly concentrated samples that are not diluted may cause distorted results, whereas samples with very low concentrations require attention to the detection limit and background signal.

(4) Storage conditions

Urinary protein samples should be stored at low temperature. For long-term storage, samples should be aliquoted and frozen to avoid repeated freeze-thaw cycles. β2-Microglobulin is sensitive to acidic urine and storage conditions; improper sample storage may lead to falsely low results.

Table 2 Common Causes of Failure in Urinary Protein Biomarker Detection and Control Strategies

8 Product Selection for Urinary Protein Biomarker Detection in Kidney Injury

9 Common Questions

9.1 Which urinary protein biomarkers should be detected after a positive urine protein dipstick result?

A urine protein dipstick is only for preliminary screening and cannot distinguish the source of proteinuria. To determine the injury level, urinary albumin, transferrin, and IgG are recommended. If IgG is markedly elevated or a post-renal source is suspected, α2-macroglobulin, urinary occult blood, and urinary sediment should be added.

9.2 What is the difference between urinary microalbumin and routine albumin detection?

Urinary microalbumin is more suitable for screening low-level albuminuria and is commonly used in studies of early glomerular permeability changes. Routine albumin detection is more suitable for higher albumin levels. Low-abundance urine samples, animal models, and early kidney injury studies are better suited to urinary microalbumin ELISA.

9.3 Can normal albumin exclude kidney injury?

No. Normal albumin cannot exclude tubular injury, interstitial injury, or proteinuria dominated by non-albumin proteins. If drug-induced kidney injury, acute kidney injury, or mixed injury is suspected, β2-microglobulin, α1-microglobulin, KIM-1, NGAL, or other markers should be added.

9.4 Does transferrin increase earlier than albumin?

It cannot be simply concluded that transferrin always increases earlier. Transferrin can serve as a complementary marker for early glomerular barrier abnormality, but results are affected by disease type, detection method, sample quality, and hematuria interference. A more reasonable approach is to analyze transferrin together with albumin and IgG.

9.5 What does increased urinary IgG indicate?

Increased urinary IgG usually indicates increased leakage of large molecular proteins and is often associated with impaired glomerular size selectivity or nonselective proteinuria. If hematuria or urinary tract inflammation is present, false elevation caused by plasma IgG contamination should first be excluded.

9.6 Does increased α2-macroglobulin always indicate severe glomerular injury?

No. α2-Macroglobulin has an extremely large molecular weight. Its elevation in urine may indicate severe barrier disruption, but it may also result from urinary tract bleeding, inflammatory exudation, or sample contamination. Interpretation must be combined with urinary occult blood, urinary red blood cells, white blood cells, and repeat sampling results.

Urinary protein biomarkers should be combined according to the detection purpose. Albumin is used for early screening, transferrin supports selectivity assessment, IgG reflects large molecular leakage, and α2-macroglobulin is used to evaluate severe barrier disruption and investigate post-renal sources.

For more related articles, please see below:

[1] Animal models of hyperuricemia and uric acid nephropathy

[2] Animal model of diabetic nephropathy