The indication to measure the absolute amount of protein excretion usually begins with the finding of a positive test for protein with the standard urinary dipstick. Another indication is the assessment of patients at increased risk for chronic kidney disease, particularly diabetic nephropathy. In most cases, the use of urine dipsticks for screening, including dipsticks specific for albuminuria, is acceptable for first detecting proteinuria. However, such dipsticks should not be used to quantify the amount of urinary protein.
Standard urinary dipstick
The standard urinary dipstick measures albumin concentration via a colorimetric reaction between albumin and tetrabromophenol blue, producing different shades of green according to the concentration of albumin in the sample.
Negative`
Trace — between 15 and 30 mg/dL
1+ — between 30 and 100 mg/dL
2+ — between 100 and 300 mg/dL
3+ — between 300 and 1000 mg/dL
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4+ — >1000 mg/dL
The urine dipstick is therefore not very accurate in assessing the severity of proteinuria since the protein concentration is a function of urine volume as well as the quantity of protein present. As an example, suppose a patient excretes 500 mg of protein per day. If the urine volume is two liters, the protein concentration will be 25 mg/dL, resulting in a trace to 1+ findings on the dipstick.
However, if the urine volume is only 500 mL, the protein concentration will be 100 mg/dL and the dipstick will read 2+. The urine dipstick is also a relatively insensitive marker for initial increases in protein excretion, not generally becoming positive until protein excretion exceeds 300 to 500 mg/day
The dipstick may also be insufficiently sensitive in multiple myeloma. Given that the dipstick primarily detects urinary albumin, it may be negative in patients with multiple myeloma who may excrete relatively large amounts of monoclonal immunoglobulin light chains. In contrast, testing the urine with sulfosalicylic acid will detect all proteins, as evidenced by the degree of turbidity.
[59] As a result, any patient with unexplained renal failure, benign urine sediment, and a negative dipstick for protein should have the urine tested with sulfosalicylic acid. A positive finding suggests the presence of non-albumin proteins in the urine, which in adults usually represents immunoglobulin light chains.
In addition to albumin, the dipstick can also detect urinary lysozyme, the production and excretion of which may be increased in patients with acute monocytic or myelocytic leukemia. Total lysozyme excretion is usually below 1 g/day but can exceed 4.5 g/day in some patients [60]. Thus, lysozyme excretion should be measured in this setting, particularly if other signs of the nephrotic syndrome (such as edema and hyperlipidemia) are absent.
28 Albuminuria dipsticks
There are also a variety of semi quantitative dipsticks, such as Clinitek Microalbumin Dipsticks and Micral-Test II test strips, which can be used to screen for albuminuria. The sensitivity and specificity of these tests range from 80 to 97 percent and 33 to 80 percent, respectively. [61]
As with standard dipsticks, these strips may be in error due to variations in urine concentrations.
As a result, these tests should only generally be used to approximate the amount of urinary protein if the ability to directly measure urinary protein excretion is not available. [62]
Quantitative measurements
The accurate measurement of protein excretion in the urine can be performed by several different techniques. The gold standard for measurement of protein excretion is a 24 hour urine collection, with the normal value being less than 150 mg/day. However, an adequate collection must be ensured. This is cumbersome for patients and physicians; thus, measurement on random specimens has become an accepted alternative method.
Urinary ratios
The preferred method of measuring urinary protein excretion in patients with proteinuria is either the albumin-to-creatinine ratio or the total protein-to-creatinine ratio (both measured in mg/dL) on a random urine specimen: [63]
With microalbuminuria, the albumin-to-creatinine ratio is used.
With significant proteinuria, either the total protein-to-creatinine ratio or the albumin-to-creatinine ratio is acceptable.
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These ratios on a random urine specimen correlate fairly closely with daily protein excretion in g/1.73 m2 of body surface area. [64] Thus, a ratio of 4.9 (as with respective urinary protein and creatinine concentrations of 210 and 43 mg/dL, respectively) represents daily protein excretion of approximately 4.9 g/1.73 m2.
The accuracy of the these ratios is related to the fortuitous occurrence that daily creatinine excretion is only slightly greater than 1000 mg (8.8 mmol)/day per 1.73 m2. However, the formula must be amended when the urinary creatinine concentration is measured in mmol/L (1 mg/dL equals 0.088 mmol/L).
Because of the accuracy and markedly increased convenience, the total protein-to-creatinine ratio or the albumin-to-creatinine ratio is preferred to the 24 hour urine collection for quantitative measurement of significant urinary protein. First morning specimens are preferred, but random daytime specimens are acceptable if first morning specimens are not available. [64] Specimens obtained in the evening or overnight appear to be least accurate. [64]
Newer high performance liquid chromatography based measurements are able to assess all intact urinary albumin, even immuno-nonreactive albumin, resulting in increased sensitivity and specificity and higher values for albumin excretion, even in normal subjects. [65]
Once it is known that the patient either has significant proteinuria (>300 to 500 mg/day) or has standard urinary dipstick positive proteinuria (1+ or greater), the measurement or monitoring of urinary protein excretion can be performed with either the albumin-to-creatinine ratio or the total protein-to-creatinine ratio on a random urine specimen. This is because proteinuria in this setting consists primarily of albumin. Thus, both ratios provide similar information.
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The K/DOQI guidelines note that the relative merits of measuring and monitoring the total protein-to-creatinine ratio versus the albumin-protein-to-creatinine ratio to detect and monitor kidney damage are unclear. However, given that albuminuria is a more sensitive marker than total protein for chronic kidney disease due to diabetes, hypertension, and glomerular diseases, they recommend, in adults, that the ratio in spot urine samples should be measured with the albumin-to-creatinine ratio. If the albumin-to-creatinine ratio is high (>500 to 1000 mg/g, which corresponds to urinary albumin excretion of >500 to 1000 mg/day), they state that total protein-to-creatinine ratio is also acceptable.
Limitations
As mentioned above, the accuracy of estimating proteinuria from a random urine specimen is diminished if creatinine excretion is substantially different from the expected value of approximately 1000 mg (8.8 mmol)/day per 1.73 m2. The total protein-to-creatinine ratio will underestimate protein excretion in a muscular man with a high rate of creatinine excretion and will overestimate the degree of proteinuria in a cachectic patient in whom muscle mass and creatinine excretion may be markedly reduced.
These limitations, however, are not necessarily clinically important with significant proteinuria since the exact degree of protein excretion is less important than the reproducibility of the test and changes with therapy.
Variability also occurs with urinary albumin excretion. This may be due to fever, heart failure, and other comorbid conditions. The measurement of at least two to three samples obtained in the same way that are collected within several days of each other should therefore be performed.
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Another potential limitation is that the total protein-to-creatinine ratio from a random urine specimen cannot be used to diagnose the benign condition, orthostatic or postural proteinuria. [66]
First morning spot urine can be obtained to help avoid this confounding effect. A normal value in the first morning spot urine and dipstick-positive proteinuria on an upright specimen is strongly suggestive of orthostatic proteinuria. [63]