A rapid and sensitive method for estimating low concentrations of albumin in human urine

Review: Detection and quantification of proteins in human urine

Extensive medical research showed that patients, with high protein concentration in urine, have various kinds of kidney diseases, referred to as proteinuria. Urinary protein biomarkers are useful for diagnosis of many health conditions – kidney and cardio vascular diseases, cancers, diabetes, infections. This review focuses on the instrumental quantification (electrophoresis, chromatography, immunoassays, mass spectrometry, fluorescence spectroscopy, the infrared spectroscopy, and Raman spectroscopy) of proteins (the most of all albumin) in human urine matrix. Different techniques provide unique information on what constituents of the urine are. Due to complex nature of urine, a separation step by electrophoresis or chromatography are often used for proteomics study of urine. Mass spectrometry is a powerful tool for the discovery and the analysis of biomarkers in urine, however, costs of the analysis are high, especially for quantitative analysis. Immunoassays, which often come with fluorescence detection, are major qualitative and quantitative tools in clinical analysis. While Infrared and Raman spectroscopies do not give extensive information about urine, they could become important tools for the routine clinical diagnostics of kidney problems, due to rapidness and low-cost. Thus, it is important to review all the applicable techniques and methods related to urine analysis. In this review, a brief overview of each technique's principle is introduced. Where applicable, research papers about protein determination in urine are summarized with the main figures of merits, such as the limit of detection, the detectable range, recovery and accuracy, when available.

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Urinary protein biomarkers are useful for diagnosis of many conditions: kidney and cardio vascular diseases, cancers.

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Liquid chromatography – mass spectroscopy is a powerful tool for urine proteomics, but used mostly in science.

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Immunoassays are widely used in both clinical and bio-analytical laboratories.

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IR and Raman spectroscopies are promising tools for diagnostics of urine due to low-cost and rapidness.

A combination of magnetic permeability detection with nanometer-scaled superparamagnetic tracer and its application for one-step detection of human urinary albumin in undiluted urine

A rapid (6.5 min) and simple one-step magnetic immunoassay (MIA) has been developed for analysis of human urinary albumin in near patient settings. Polyclonal rabbit anti-human albumin was used as a capture antibody and monoclonal mouse anti-human albumin as a detection antibody in a two-site immunometric assay requiring no additional washing procedures. The polyclonal anti-human albumin was conjugated to silica microparticles (solid phase) and the monoclonal antibody to dextran-coated nanoscaled superparamagnetic particles (tracer). Quantification of human albumin in undiluted urine was performed by adding 2 microL urine to a measuring vial containing solid-phase, superparamagnetic tracer and reaction buffer and then inverting the vial by hand for 20 s. The measuring vial was allowed to stand for 6 min prior to detection, in order for the solid-phase sediment to form at the bottom of the vial. Lastly, the measuring vial was placed into a magnetic permeability detector, which measured the enrichment of superparamagnetic tracer in the sediment due to complex formation with human albumin. Total analysis time was 6.5 min. A linear response was obtained for 0-400 mg/L albumin with a detection limit of 5 mg/L. The total coefficient of variation (CV) was 11% calculated from four consecutive runs on a urine sample containing 11.1 mg/L human albumin during 3 consecutive days. Human urinary albumin analysis was performed on 149 patient samples using the MIA technique and the obtained results showed good correlation with the hospital immunoturbidimetric reference method (y = 1.004x + 4.01, R2 = 0.978, N = 149) and a commercially available point of care albumin analysis provided by HemoCue Inc. (y = 0.98x + 5.8, R2 = 0.833, N = 90).

Measurement of albumin in nasal lavage using the technique of chemiluminescent immunoassay

A method has been developed that uses chemiluminescent acridinium esters rather than radioactive iodine in an immunoassay for albumin. Albumin is a protein derived from plasma sources found in nasal fluids. As such, it can be used as an important marker of plasma exudation in experimental rhinology. The assay was developed as an alternative to radioimmunoassay for a number of reasons: chemiluminescent assays do not require radioactive materials and the complex safety related aspects of their usage; the assays are easy to perform and do not require expensive equipment; and the assay is capable of the sensitivity required to detect very small changes in albumin concentrations in nasal fluids. The assay measures albumin with a sensitivity of 1.2 ng/ml. The working range of the assay is 1.0-23.7 micrograms/ml with an interassay coefficient of variation < or = 15%. This working range encompasses the range of albumin usually found in nasal lavage samples from normal volunteers.

Water-soluble conductive polymer homogeneous immunoassay (SOPHIA). A novel immunoassay capable of automation

Conductive polymers are extensively conjugated macromolecules able to conduct electricity in their doped state and having a UV-visible spectrum which undergoes important chromatic modifications when subjected to pH changes or to oxido-reductive processes. This article describes a novel homogeneous immunoassay in which a water-soluble conductive polymer is used as the label. When antigen-antibody binding occurs, the local pH near the complex is modified. Such a pH change is in turn able to induce modifications in the absorbance at a characteristic wavelength of a conductive polymer covalently linked to either the antigen or the antibody. Consequently, the extent of tracer binding can be directly monitored by photometry during incubation. We present examples which validate the concept and exemplify its applicability in quantitative competitive immunoassays for human C-reactive protein and human serum albumin, as performed in a Cobas-Mira automated analyzer.

Permselectivity in thin membrane nephropathy

The glomerular permselectivity to polydisperse neutral dextrans was compared in 6 patients with thin membrane nephropathy (TMN) and 10 healthy controls. Despite having normal renal hemodynamics and minimal proteinuria, the patients with TMN had significantly increased fractional clearance of neutral molecules with Stokes radius > 42 A. Conventional theories of glomerular barrier size selectivity cannot fully explain these data since they would predict that our patients would have had nephrotic range proteinuria.

Preservation of renal haemodynamic response to an oral protein load in non-insulin-dependent diabetes mellitus

Glomerular Filtration Rate (GFR) and Effective Renal Plasma Flow (ERPF) were determined, for 2 h prior to and 3 h following the ingestion of a 1.2 g kg-1 meat meal, in seven normotensive normoalbuminuric Type 2 diabetic patients exhibiting good glycaemic control (fasting plasma glucose (mean +/- SD): 7.2 +/- 2.0 mmol l-1; glycosylated haemoglobin: 8.1 +/- 1.7%) and in nine normal subjects selected for similar basal GFR values. Baseline GFR and ERPF (corrected to 1.73 m2 surface area) were 83 +/- 10 and 410 +/- 76 ml min-1 for the Type 2 diabetic patients and 86 +/- 11 and 405 +/- 113 ml min-1 for the normals. GFR increased by 38 +/- 8 and 32 +/- 15% in the diabetic patients and normals, to 108 +/- 25 and 105 +/- 26 ml min-1 (p < 0.01 vs baseline). Peak ERPF was 501 +/- 127 and 476 +/- 119 ml min-1 for the two respective groups (p < 0.01 vs baseline). Filtration fractions at peak GFR and EPRF values were unchanged from baseline for either groups. Fractional clearance of albumin for the Type 2 diabetic patients was unaltered by protein ingestion. Therefore, protein ingestion in Type 2 diabetes, as in normals, results in an acute elevation of GFR. Absolute and incremental changes in GFR were identical for the two groups. These data demonstrate a preserved capacity for renal vasodilatation in Type 2 diabetic patients despite their greater chronological age.

Impact of initial treatment on renal function in newly-diagnosed type 2 (non-insulin-dependent) diabetes mellitus

The impact of improved glycaemic control on renal function in newly-presenting Type 2 (non-insulin-dependent) diabetic patients has not been adequately researched. Consequently, glomerular filtration rate and effective renal plasma flow and urinary albumin excretion rates were determined in 76 subjects (age (mean (SD)): 54 (9.5) years; 50 male) of an original cohort of 110 newly-presenting normotensive non-proteinuric Type 2 diabetic patients following 6 months treatment with diet alone (n = 42) or with oral hypoglycaemic agents (n = 34). Significant reductions were observed in (presentation vs 6 months): body mass index (p < 0.001); fasting plasma glucose (p < 0.001); glycated haemoglobin (HbA1) (p < 0.001); systolic blood pressure (p < 0.01); and diastolic blood pressure (p < 0.001). Glomerular filtration rate declined from 117 (22) to 112 (21) ml.min-1 (p < 0.01), with unchanged effective renal plasma flow (534 (123) vs 523 (113) ml.min-1) and filtration fraction (22.4 (3.0) vs 21.8 (3.4)%). Albumin excretion rate (median (range)) declined from 1.1 (0.1-34.7) to 0.5 (0.1-29.9) micrograms.min-1 (p < 0.01). Changes in glomerular filtration rate (delta values) were inversely correlated with presentation values (p < 0.001), and positive relationships were observed with delta effective renal plasma flow (p < 0.01), and delta glycated haemoglobin (p < 0.05). Type 2 diabetic patients with glomerular filtration rate values at presentation over 120 ml.min-1 demonstrated significant reduction in glomerular filtration rate (n = 31; p < 0.001), whilst those with original values less than 120 ml.min-1 remained unchanged (n = 45).(ABSTRACT TRUNCATED AT 250 WORDS)

Renal hemodynamics in newly presenting non-insulin dependent diabetes mellitus

Little information is available regarding renal hemodynamics in non-insulin dependent diabetics (NIDDMs), despite their numerical significance to renal support programs. Therefore, simultaneous determination of glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) (expressed to 1.73 m2 surface area) was performed in 110 newly presenting normotensive NIDDMs [mean (SD) age: 52.5 (10.1) years] and compared with 32 normal subjects of similar age range [age: 52.2 (11.3) years]. Mean (SD) GFR, ERPF and filtration fraction (FF) for the NIDDMs were: 117 (22) ml/min (range: 74 to 178), 534 (123) ml/min and 22.4 (3.0)%, and for the normals: 95 (12) ml/min (range: 75 to 119), 472 (70) ml/min and 20.2 (2.2%), respectively. As a group, NIDDMs demonstrated significantly greater GFR (P less than 0.001), ERPF (P less than 0.005) and FF (P less than 0.001). GFR values above 140 ml/min were observed in 16% of the NIDDMs, while 45% had GFRs in excess of mean +/- 2 SD of the normals. NIDDMs demonstrated a positive relationship for GFR with ERPF (P less than 0.001) and an inverse association for both GFR and ERPF with age (P less than 0.001). Multivariable regression analysis revealed ERPF as the strongest explanatory variable for GFR in the NIDDMs (P less than 0.001), followed by age (P less than 0.01). Significant contributions to the regression analysis were not observed for body mass index, systolic or diastolic blood pressures, glycosylated hemoglobin nor fasting levels of plasma glucose, cholesterol, HDL-cholesterol, LDL-cholesterol and triglycerides. Urinary albumin excretion rates were not correlated with GFR, ERPF or FF. Microalbuminuria was detected in 7% of the NIDDMs in this study.(ABSTRACT TRUNCATED AT 250 WORDS)

Laser-induced fluorometric determination of albumin using longwave absorbing molecular probes

A novel fluorescence assay for HSA is described. It is based on longwave-absorbing probes that selectively bind to HSA to form fluorescent complexes. The two probes reported here (Albumin Blue 633 and Albumin Blue 670) are tailored to match the lines of the 633-nm HeNe laser and the 670-nm diode laser, respectively. In both cases, the strong laser-induced fluorescence of the HSA/probe complexes makes the assay sensitive to HSA at trace levels. Detection limits of 0.2 mg/liter were obtained. In addition to its sensitivity, the assay is highly selective for HSA in that the response to other serum proteins is weaker by a factor of at least 100. It also works well on urine, and no significant interferences could be located. Potential interferents are bovine serum albumin and some detergents. Parameters of the probe--HSA interaction were obtained from a Scatchard evaluation. The high specificity and sensitivity of the assay are discussed in terms of molecular geometry of the probes. A photophysical mechanism that leads to the effect is proposed. The assay presents a promising alternative to immunological determinations of HSA since it is less expensive and much more quickly performed.