Urinalysis in the diagnosis of renal disease

INTRODUCTION —

Patients with renal disease have a variety of different clinical presentations. Some have symptoms that are directly referable to the kidney (gross hematuria, flank pain) or to extrarenal sites of involvement (edema, hypertensive, signs of uremia). Many patients, however, are asymptomatic and are noted on routine examination to have an elevated plasma creatinine concentration or an abnormal urinalysis.

Once renal disease is discovered, the presence or degree of renal dysfunction is assessed and the underlying disorder is diagnosed. Although the history and physical examination can be helpful, the most useful information is initially obtained from estimation of the glomerular filtration rate (GFR) and examination of the urinary sediment.

Estimation of the glomerular filtration rate (GFR) is used clinically to assess the degree of renal impairment and to follow the course of the disease. However, the GFR provides no information on the cause of the renal disease. This is achieved by the urinalysis and, if necessary, radiologic studies and/or renal biopsy.

URINALYSIS — The major noninvasive diagnostic tool

available to the clinician is the urinalysis. Although examination of the urine can also provide some information about disease

severity, such a direct relationship between the urinalysis and severity is not always present.

In a patient with acute glomerulonephritis, for example,

Unormalization of the urinalysis represents resolution of the active inflammatory processU.

However, this can reflect either recovery or healing with irreversible glomerular scarring and nephron loss. In this setting, repeat renal biopsy may be required to accurately estimate the status of the

renal disease [1] .

Despite these potential limitations, a complete urinalysis should be performed in all patients with renal disease. The specimen should be examined within 30 to 60 minutes of voiding; a midstream specimen is adequate in men, but the external genitalia should first be cleaned in women to avoid contamination with vaginal secretions. The urine should be centrifuged at 3000 rpm for three to five minutes, and the supernatant then poured into a separate tube. A small amount of sediment should be placed on a slide, while the supernatant should be tested for color (particularly for color suggesting the presence of heme pigments), protein, pH, concentration, and glucose.

Color —

Normal urine is clear and light yellow in color; it is

lighter when dilute and darker when concentrated, such as after an overnight water restriction.

The urine may also have a variety of other colors, including shades of red or brown. The intermittent excretion of red to brown urine is observed in a variety of clinical settings [2,3] .

The initial step in the evaluation of this problem is centrifugation of the urine to see if the red color is in the urine sediment or the supernatant

Hematuria is responsible if the red color is seen only in the urine sediment, with the supernatant being clear

If, on the other hand, it is the supernatant that is red, then the supernatant should be tested for heme with a urine dipstick.

UA red supernatant that is negative for heme is a rare finding that can be seen in several conditions

These include use of Uthe bladder analgesic phenazopyridineU or a variety of other medications, food dyes, the ingestion of beets in susceptible subjects, and porphyria

UA red supernatant that is positive for heme is usually due to myoglobinuria or hemoglobinuriaU.

UHemoglobinuria and myoglobinuria can be usually be distinguished by looking at the plasma which is red with hemoglobinuria and its normal color with myoglobinuria.

UFalse positive heme reactions may be seen if semen is present in the urine, if the urine is alkaline (pH >9), or contaminated with oxidizing agents used to clean the perineumU [4] .

Rarely, the urine has other colors.

These include:

White due to pyuria, phosphate crystals, chyluria [5,6] , or propofol [7] .

Green due to the administration of methylene blue [8] , propofol [9-11] , or amitriptyline).

Black due to hemoglobinuria [12,13] or ochronosis, most often due to alkaptonuria, which is also called black urine disease.

The black urine in ochronosis results from urinary excretion of homogentisic acid (HGA).

The black color may only be apparent after the urine stands for some time, permitting oxidation of HGA. (See "Disorders of tyrosine metabolism", section on Alkaptonuria).

Purple due to bacteriuria in patients with urinary catheters. (See "Urinary tract infection associated with indwelling bladder catheters", section on Purple urine bag syndrome).

In addition, other colors can occur in children with inborn errors of metabolism

Protein —

The urine dipstick primarily detects albumin but not other proteins, such as immunoglobulin light chains. This test is highly specific, but not very sensitive for the detection of proteinuria; it becomes positive only when protein excretion exceeds 300 to 500 mg/day.

Thus, the regular urine dipstick is an insensitive method to detect microalbuminuria, which is the earliest clinical manifestation of diabetic nephropathy and is associated with increased cardiovascular risk in patient with and without diabetes.

U In type 1 diabetes, the development of a positive dipstick for albumin is a relatively late event, occurring at a time when there is already substantial structural injury.

There are also a variety of dipsticks that can be used to test for microalbuminuria, such as :

o Clinitek Microalbumin Dipsticks and o Micral-Test II test strips

The semiquantitative categories on the dipsticks should be used with caution and only as a rough guide since, at a given of albumin excretion, the albumin concentration will vary with the urine volume. A dilute urine, for example, will underestimate the degree of proteinuria, while a highly concentrated urine may have a 3+ response on the regular dipstick but not be indicative of heavy proteinuria.

UFalse-positive results are common with many iodinated radiocontrast agents [14] .

UThus, the urine should not be tested for protein with the dipstick for at least 24 hours after a contrast study.

Sulfosalicylic acid test —

In contrast to the urine dipstick, SSA detects all proteins in the urine [15] .

This characteristic makes the SSA test particularly useful in older patients who present with acute renal failure, a benign urinalysis, and a negative or trace dipstick.

In this setting, myeloma kidney, in which immunoglobulin light chains form casts that obstruct the tubules, must be excluded. A significantly positive SSA test in conjunction with a negative dipstick usually indicates the presence of nonalbumin proteins in the urine, most often immunoglobulin light chains.

Similar to the urine dipstick, radiocontrast agents can cause false positive SSA results [3] .

The sulfosalicylic acid (SSA) test is performed by mixing one part urine supernatant (eg, 2.5 mL) with three parts 3 percent sulfosalicylic acid, and grading the resultant turbidity according to the following schema (the numbers in parentheses represent the approximate protein concentration) [2] :

U 0 = no turbidity (0 mg/dL)

U trace = slight turbidity (1 to 10 mg/dL)

U 1+ = turbidity through which print can be read (15 to 30 mg/dL)

U 2+ = white cloud without precipitate through which heavy black lines on a white background can be seen (40 to 100 mg/dL)

U 3+ = white cloud with fine precipitate through which heavy black lines cannot be seen (150 to 350 mg/dL)

U 4+ = flocculent precipitate (>500 mg/dL)

Quantitative measurement of urinary protein excretion —

Most patients with persistent proteinuria should undergo a quantitative measurement of protein excretion. This can be accomplished by a 24-hour urine measurement; however, collecting these specimens may be cumbersome in ambulatory care settings.

An alternative and preferred method is calculating the total protein-to-creatinine or albumin-to-creatinine ratio (mg/mg) on a urine sample [16-18] .

With elevated urinary protein levels, it is important to understand how to differentiate between relatively benign (eg, orthostatic proteinuria) or common causes of proteinuria (eg, diabetic proteinuria) and uncommon causes that require nephrology consultation.

Hydrogen ion concentration —

The urine hydrogen ion concentration, measured as the pH, reflects the degree of acidification of the urine.

The urine pH ranges from 4.5 to 8.0, depending upon the systemic acid-base balance.

The major clinical use of the urine pH occurs in patients with metabolic acidosis.

The appropriate response to this disorder is to increase urinary acid excretion, with the urine pH falling below 5.3 and usually below 5.0.

A higher value may indicate the presence of one of the forms of renal tubular acidosis.

Distinction between the various types of RTA can be made by measurement of the urine pH and the fractional excretion of bicarbonate at different plasma bicarbonate concentrations.

The diagnostic use of the urine pH requires that the urine be sterile. Infection with any pathogen that produces urease, such as Proteus mirabilis, can result in a urine pH above 7.0 to 7.5.

Osmolality and specific gravity —

The solute concentration of the urine (or other solution) is a function of the number of solute particles per unit volume; it is most accurately measured by the osmolality of the solution.

The plasma osmolality is maintained within a very narrow range (approximately 285 mosmol/kg), principally because the kidney is able to excrete urine with an osmolality markedly different from that of plasma

SINCE THE URINARY CONCENTRATION VARIES MARKEDLY BASED UPON VOLUME STATUS, THE URINE OSMOLALITY IS USEFUL ONLY WHEN CORRELATED WITH THE CLINICAL STATE. THIS MEASUREMENT IS MOST USEFUL IN THE DIAGNOSIS OF PATIENTS WITH HYPONATREMIA, HYPERNATREMIA, AND POLYURIA.

If an osmometer is unavailable, the concentration of the urine can be assessed by measuring the specific gravity, which is defined as the weight of the solution compared with that of an equal volume of distilled water. The specific gravity generally varies with the osmolality. However, the presence of large molecules in the urine, such as glucose or radiocontrast media, can produce large changes in specific gravity with relatively little change in osmolality.

Glucose —

The presence of glucose in the urine as detected semiquantitatively with a dipstick may be due to either the inability of the kidney to reabsorb filtered glucose in the proximal tubule despite normal plasma levels (renal glucosuria) or urinary spillage because of abnormally high plasma concentrations.

IN PATIENTS WITH NORMAL RENAL FUNCTION, SIGNIFICANT GLUCOSURIA DOES NOT GENERALLY OCCUR UNTIL THE PLASMA GLUCOSE CONCENTRATION IS ABOVE 180 MG/DL (10 MMOL/L).

Renal glucosuria can occur as an isolated defect but is more commonly observed in association with additional manifestations of proximal dysfunction, including :

o hypophosphatemia,o hypouricemia,o renal tubular acidosis, and o aminoaciduria

This constellation is called the Fanconi syndrome and may result from a variety of disorders, particularly multiple myeloma.

The use of urinary glucose levels to screen for and monitor diabetes mellitus is limited for a number of reasons. These include the relative insensitivity of the measurement (since moderate hyperglycemia is required before a positive test is obtained); its dependence upon the urine volume; and its value which reflects the mean plasma glucose concentration and not the level at the time of measurement

Dipstick detection of hematuria and pyuria —

Microscopic hematuria may be discovered incidentally when heme (either red blood cells or hemoglobin) is detected on a dipstick. Dipsticks for hemoglobin detect 1 to 2 RBCs per high powered field and are therefore at least as sensitive as urine sediment examination, but result in more false positive tests.

False positive results may occur with alkaline urine with a pH greater than 9, contamination with oxidizing agents used to clean the perineum, and semen present in the urine after ejaculation.

By comparison, false negative tests are unusual; as a result, a negative dipstick reliably excludes abnormal hematuria [19] .

Although red cells may be lysed in dilute urine, the hemoglobin that is released will be detected by the dipstick.

Dipsticks may also detect leukocyte esterase and nitrite, the former corresponding to pyuria and the latter to Enterobacteriaceae which convert urinary nitrate to nitrite.

Although this test is a simple and inexpensive screen for urinary tract infection, it may also detect pyuria not associated with infection.

Significant causes of sterile pyuria include :

o interstitial nephritis,o renal tuberculosis, ando nephrolithiasis

URINE SEDIMENT —

Hematuria and pyuria by dipstick may be useful as a screening test.

However, a microscopic examination of the urine sediment is essential in the evaluation of renal disease, as it permits detection of elements which cannot be found by dipstick alone (eg, red and white blood cells and casts, and epithelial cells or casts).

SMALL AMOUNTS OF CRYSTALS, BACTERIA, CELLS, OR CASTS MAY BE OBSERVED IN HEALTHY INDIVIDUALS.

In a normal patient, for example, one high power field may contain 0 to 4 white blood cells and 0 to 2 red blood cells, and one cast may be observed in 10 to 20 low powered fields [20] .

In addition, crystals of uric acid, calcium oxalate, or phosphate may occasionally be seen.

The specimen is ideally examined within 30 to 60 minutes of voiding. The urine should be centrifuged at 3000 rpm for three to five minutes, most of the supernatant poured out, and the pellet resuspended with gentle shaking of the tube.

A small amount of the resuspended sediment is poured on the slide. The urine sediment examination should be performed by a clinician trained in urine microscopy, as the diagnostic yield compared to a laboratory urinalysis may be substantially greater [21] .

Crystalluria —

Whether crystals form in the urine depends upon a variety of factors, including :

o the degree of supersaturation of constituent molecules,o the urine pH, ando the presence of inhibitors of crystallization

Many different forms may be observed in normal patients and in those with defined disorders:

Uric acid crystals —

Uric acid crystals as well as amorphous urates are observed in acid urine, a milieu which favors the conversion of the relatively soluble urate salt into the insoluble uric acid

Calcium phosphate or calcium oxalate crystals —

The formation of calcium oxalate crystals is not dependent upon the urine pH, while calcium phosphate crystals only form in a relatively alkaline urine

Cystine crystals —

Cystine crystals, with their characteristic hexagonal shape, are diagnostic of cystinuria

Magnesium ammonium phosphate crystals —

Magnesium ammonium phosphate (struvite) and calcium carbonate-apatite are the constituents of struvite stones

Normal urine is undersaturated with ammonium phosphate and struvite stone formation occurs only when ammonia production is increased and the urine pH is elevated to decrease the solubility of phosphate.

BOTH OF THESE REQUIREMENTS MAY BE MET WHEN URINARY TRACT INFECTION OCCURS WITH A UREASE-PRODUCING ORGANISM, SUCH AS PROTEUS OR KLEBSIELLA.

ALTHOUGH THE OBSERVATION OF CRYSTALS IN THE URINE IS MOST FREQUENTLY OF LITTLE DIAGNOSTIC IMPORTANCE, THERE ARE SEVERAL NOTABLE EXCEPTIONS.

These include :

U1.. the presence of cystine or ammonium magnesium phosphate crystals (as mentioned above),

U2.. the combination of acute renal failure and calcium oxalate crystals (a setting consistent with ethylene glycol ingestion), and

U3.. the presence of a larger number of uric acid crystals occurring in association with acute renal failure (consistent with tumor lysis syndrome).

Bacteria —

The presence of bacteria in a urine sediment is most frequently

due to contamination of the specimen upon collection.

Although normal urine is sterile, asymptomatic bacteriuria is increasingly recognized but is usually not treated.

Cells —

The cellular elements found in the urinary sediment include :

o red blood cells, o white blood cells, and o epithelial cells

Infrequently :

o tumor cells may also be observed, thereby suggesting the diagnosis of genitourinary malignancy (eg, bladder cancer) and/or

o infiltration of the renal parenchyma with malignant cells (eg, lymphoma).

Hematuria —

The presence of hematuria can be benign or reflect serious underlying disease

HEMATURIA MAY BE GROSSLY VISIBLE OR MICROSCOPIC.

Microscopic hematuria is commonly defined as the presence of more than 2 red blood cells per high powered field in a spun urine sediment

The color change in gross hematuria does not necessarily reflect a large degree of blood loss since as little as 1 mL of blood per liter of urine can induce a visible color change.

As previously mentioned, the intermittent excretion of red to brown urine can be observed without red blood cells.

Hematuria may be transient or persistent. Transient hematuria is relatively common in young subjects and is not usually indicative of disease in this population.

UHOWEVER, EVEN TRANSIENT HEMATURIA CAN REPRESENT UNDERLYING MALIGNANCY IN PATIENTS OVER AGE 50.

Transient hematuria can also occur with urinary tract infection(eg, cystitis or prostatitis). This is typically accompanied by pyuria and bacteriuria and patients may often complain of dysuria.

Persistent hematuria should always be evaluated. Among the more common pathologic causes are :

o kidney stones, o malignancy, and o glomerular disease

THUS, AN IMPORTANT ASPECT OF EVALUATION IS DETERMINING WHETHER THE HEMATURIA IS GLOMERULAR IN ORIGIN OR EXTRAGLOMERULAR.

UFindings strongly suggestive of glomerular disease are :

o red cell casts,o proteinuria, and o dysmorphic red cells, particularly acanthocytes

The small number of red cells in normal urine are also dysmorphic, suggesting a glomerular origin [23] .

Pyuria —

White cells are slightly larger than red cells and can be identified by their characteristic granular cytoplasm and multilobed nuclei (since most are neutrophils

UInfection is the most common cause of pyuria alone;U the routine urine culture may be negative with tuberculous infection.

UPYURIA HAS LESS DIAGNOSTIC VALUE IF IT OCCURS IN ASSOCIATION WITH OTHER :

o UCELLULAR CASTS, o UADDITIONAL CELLULAR ELEMENTS, AND/OR

PROTEINURIA

In addition to neutrophils, eosinophils and lymphocytes may also be seen in the urine.

These cells can be identified by a Wright's stain of the sediment.

Although it has been proposed that the finding of eosinophiluria is relatively specific and might be diagnostic of acute interstitial nephritis, the diagnostic accuracy of urinary eosinophils is uncertain

Urinary lymphocytes may be observed in disorders associated with infiltration of the kidney by lymphocytes, such as chronic tubulointerstitial disease. ("Renal disease in sarcoidosis").

Epithelial cells —

Epithelial cells may appear in the urine after being shed from anywhere within the genitourinary tract.However, only renal tubular cells are diagnostically significant. Renal tubular cells are 1.5 to 3 times larger than white cells and

contain a round, large nucleus. SINCE IT IS DIFFICULT TO DISTINGUISH RENAL TUBULAR CELLS FROM LOWER URINARY TRACT CELLS, THE PRESENCE OF EPITHELIAL CELLS IN CASTS IS THE ONLY RELIABLE FINDING TO INDICATE A RENAL ORIGIN OF THE CELL.

Although an occasional finding of an epithelial cell cast is normal, increased numbers suggest a number of disorders, including :

o acute tubular necrosis, o pyelonephritis, and the o nephrotic syndrome

Novel assays —

Glomerular epithelial cells, or podocytes, are involved in maintenance of the filtration barrier in the glomerular capillary tuft, and are the principal site of injury or genetic abnormality in certain glomerulopathies.

Podocytes are normally absent or present in only very small quantities in the urine of individuals without kidney disease, or with inactive glomerular diseases [24] , but excretion appears to increase with active disease, and decrease with treatment [24-26] .

It has been suggested that podocyturia may be a better marker of ongoing glomerular damage than proteinuria, but the test is not yet available clinically [27] .

Casts —

Casts conform to the shape of the renal tubule in which they

formed and are therefore cylindrical with regular margins. All casts have an organic matrix composed primarily of Tamm-Horsfall mucoprotein.

Many different types of casts may be observed. Some can be found in normal individuals, while others are diagnostic of significant renal disease [28] .

The observation of cells within a cast is highly significant since their presence is diagnostic of an intrarenal origin.

Hyaline casts —

Hyaline casts, which are only slightly more refractile than water, Uare not indicative of disease and are primarily observed U :

o with small volumes of concentrated urine oro with diuretic therapy

they may occur at a frequency of 10 casts per high powered field.

Red cell casts —

The finding of red cell casts, even if only one is seen, is virtually

diagnostic of glomerulonephritis or vasculitis

White cell casts —

The presence of white cell casts and pyuria alone is most consistent with :

o a tubulointerstitial disease oro acute pyelonephritis

They may also be observed with many glomerular disorders.

Epithelial cell casts —

Acute tubular necrosis and acute glomerulonephritis, disorders in which epithelial cells are desquamated, may be associated with epithelial cell casts

Fatty casts —

Among patients with significant proteinuria, the degeneration of cells within epithelial casts may result in a characteristic "Maltese cross" appearance and a fatty cast

These droplets are composed of cholesterol esters and cholesterol, which may also be observed free in the urine.

Granular casts —

Granular casts, which are observed in numerous disorders, represent degenerating cellular casts or aggregated proteins

Waxy casts —

Waxy casts are thought to be the last stage of the degeneration of a granular cast Since this degenerative process is probably slow, it is most likely observed in nephrons with very diminished flow.

Waxy casts are therefore most consistent with the presence of advanced renal failure.

Broad casts —

AS WITH WAXY CASTS, BROAD CASTS, WHICH ARE WIDER THAN OTHER CASTS AND TEND TO HAVE A GRANULAR OR WAXY APPEARANCE, ARE THOUGHT TO FORM IN THE LARGE TUBULES OF NEPHRONS WITH LITTLE FLOW. THEY ARE MOST OFTEN OBSERVED IN PATIENTS WITH ADVANCED RENAL FAILURE.

PATTERNS —

The diagnostic value of the urinalysis in the patient with renal disease lies in the association between different patterns of urinary findings and different renal diseases.

In many cases, the urinary findings point toward one or only a few disorders

1.. HEMATURIA WITH RED CELL CASTS, DYSMORPHIC RED CELLS, PROTEINURIA, AND/OR LIPIDURIA —

This constellation of findings is virtually diagnostic of glomerular disease or vasculitis

However, the absence of these pathognomonic changes in patients with hematuria does not exclude these disorders.

2.. Multiple granular and epithelial cell casts with

free epithelial cells —

These findings are strongly suggestive of acute tubular necrosis in a patient with acute renal failure, although their absence does not exclude this diagnosis

In this setting, ischemic or toxic injury to the tubular epithelial cells can lead to cell sloughing into the tubular lumen due either to cell death or to defective cell-to-cell or cell-to-basement membrane adhesion [29] .

In addition to acute tubular necrosis, similar urinary abnormalities can also be induced by marked hyperbilirubinemia alone (plasma bilirubin concentration usually above 8 to 10 mg/dL or 136 to 170 µmol/L); how this occurs is not clear [30] .

3.. Pyuria with white cell and granular or waxy casts and no or mild proteinuria —

This constellation is suggestive of tubular or interstitial disease or urinary tract obstruction

White cells and white cell casts can also be seen in acute glomerulonephritis, particularly postinfectious glomerulonephritis; in this setting, however, there are also other signs of glomerular disease, such as hematuria, red cell casts, and proteinuria.

4.. Hematuria and pyuria with no or variable casts (excluding red cell casts) —

These findings may be seen in acute interstitial nephritis, glomerular disease, vasculitis, obstruction, and renal infarction.

Eosinophiluria may also be seen with acute interstitial nephritis, but the absence of this finding does not exclude the diagnosis.

5.. Hematuria alone —

The significance of isolated hematuria (ie, without other cellular elements or casts, including red cell casts) varies with the clinical setting.

It is suggestive of vasculitis or obstruction In the patient with acute renal failure, and of urolithiasis in the patient with flank pain.

It can also be found with mild glomerular disease (particularly postinfectious glomerulonephritis, IgA nephropathy, thin basement membrane disease, and hereditary nephritis), polycystic kidney disease, and with extrarenal disorders such as tumors, and prostatic disease.

6.. Pyuria alone —

Assuming no contamination with vaginal secretions (which is unlikely if there are no large vaginal epithelial cells in the sediment), pyuria alone is usually indicative of urinary tract infection (including tuberculosis).

Sterile pyuria suggests some form of tubulointerstitial disease, such as analgesic nephropathy.

7.. Normal or near-normal (few cells with little or no casts or proteinuria; hyaline casts are not anabnormal finding) —

In Patients With Acute Renal Failure, A Relatively Normal Urinalysis Suggests Prerenal Disease, Urinary Tract Obstruction, Hypercalcemia, Myeloma Kidney (Although The SSA Test Should Be Markedly Positive), Some Cases Of Acute Tubular Necrosis, Or A Vascular Disease With Glomerular Ischemia But Not Infarction (Scleroderma, Atheroemboli [Which Are Irregularly Shaped And Do Not Completely Occlude Vessels], And Rare Cases Of Polyarteritis Nodosa Affecting The Renal Arteries But Not The Glomeruli).

WITH CHRONIC RENAL DISEASE, DISORDERS THAT SHOULD BE CONSIDERED INCLUDE:

o prerenal disease (as with congestive heart failure), o urinary tract obstruction, o benign nephrosclerosis, and o tubular or interstitial diseases