The urine specimen is analyzed in 3 main parts.
Gross visual examination
Normal urine color is due to the presence of a pigment called urochrome. Urine color varies based on the urine concentration and chemical composition. Normal urine can vary from pale light yellow to a dark amber color. Highly concentrated urine has a darker yellow appearance. This may be seen in patients who are volume depleted. In contrast, dilute urine has a lighter yellow appearance. This may be seen in patients with diabetes insipidus, due to impaired urine concentrating ability. Urine color may vary due to certain medications, foods, and medical conditions.
Red urine may indicate the following:
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Foods – Beets, blackberries, rhubarb
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Drugs – Propofol, chlorpromazine, thioridazine, Ex-lax
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Medical conditions – Urinary tract infections (UTIs), nephrolithiasis, hemoglobinuria (rhabdomyolysis), porphyrias (urine color, port wine)
Orangeurine may indicate the following:
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Foods – Carrot, vitamin C
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Drugs – Rifampin, phenazopyridine
Greenurine may indicate the following:
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Food – Asparagus
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Drugs – Vitamin B, methylene blue, propofol, amitriptyline
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Medical condition – UTI
Blueurine may indicate the following:
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Drugs – Methylene blue, indomethacin, amitriptyline, triamterene, cimetidine (intravenous), promethazine (intravenous)
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Medical condition – Blue diaper syndrome (also known as tryptophan malabsorption)
Purpleurine may indicate the following:
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Medical condition – Bacteriuria in patients with urinary catheters (purple urine bag syndrome)
Brownurine may indicate the following:
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Food – Fava beans
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Drugs – Levodopa, metronidazole, nitrofurantoin, primaquine, chloroquine, methocarbamol, senna
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Medical conditions – Gilbert syndrome, tyrosinemia, hepatobiliary disease
Blackurine may indicate the following:
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Medical conditions – Alkaptonuria, malignant melanoma
Whiteurine may indicate the following:
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Drug – Propofol
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Medical conditions – Chyluria, pyuria, phosphate crystals
Urine clarity or turbidity refers to how clear the urine is. It is determined by substances in urine, such as the amount of cellular debris, casts, crystals, bacteria, or significant proteinuria. vagin*l discharge, sperm, and prostatic secretions may also influence the outcome. Urine clarity is typically classified as clear, mildly cloudy, cloudy, or turbid.
Chemical examination/urine dipstick
In most individuals, urine pH is usually lower, representing a slightly acidic environment. This results from the obligate renal H+ ion excretion, due to the normal daily average endogenous acid production of 1 mEq/kg required to maintain acid-base balance in the body. [1] Therefore, any abnormalities in the acid-base balance in the body has a direct effect on urinary pH levels. Diet can also affect urinary pH levels. Cranberries and high-protein diets create a more acidic urinary environment, whereas citrus fruits and low-carbohydrate diets create a more alkaline urine environment. [2]
Urinary pH levels are particularly useful in the evaluation of stones, infection, and renal tubular acidosis (RTA). For example, in a patient with nephrolithiasis, the urinary pH level is helpful when trying to distinguish between different types of calculi. Calcium oxalate/calcium phosphate, magnesium-ammonium phosphate, and staghorn calculi are associated with alkaline urine. Conversely, uric acid and cystine calculi are associated with acidic urine (although pH is more important for treatment than formation of cystine stones). Also, patients with a UTI due to urea-splitting organisms, such as proteus and klebsiella, typically have alkaline urine. Urinary pH can also help to distinguish between the different types of RTA.
Additionally, urinary pH levels can be useful in gauging the response to treatment in patients with rhabdomyolysis or medication/drug overdoses. As stated, alkaline urine is associated with calcium oxalate/calcium phosphate calculi and struvite (magnesium ammonium phosphate) calculi. Acidic urine is associated with uric acid calculi and cystine calculi.
Specific gravity is a measurement of urine concentration and is representative of the kidney’s ability to concentrate urine. The specific gravity is a comparison of the amount of solutes in urine as compared with pure water. [2] Specific gravity may also be used as a rough estimate of urine osmolality. For each rise in the specific gravity by 0.001 above 1, the urine osmolality increases by about 30-35 mosmol/kg. For example, a urine specific gravity of 1.010 usually corresponds to a urine osmolality of about 300-350 mosmol/kg. However, in the setting of substances such as glucose and radiocontrast media, the specific gravity is increased more than the urine osmolality. [3]
Often specific gravity is reflective of hydration status; however, it can be inaccurate. Low specific gravity is seen in patients with impaired urinary concentrating ability (eg, diabetes insipidus, sickle cell nephropathy, acute tubular necrosis). A specific gravity of 1.003 or less is indicative of maximally dilute urine. In addition, low values may be seen due to glucose, urea, or alkaline urate. High values may be due to significant amounts of protein or ketoacids. [1]
Glucose present in the urine is termed glucosuria. Most commonly, this indicates diabetes mellitus but is also often seen in pregnancy. [1] It is due to either a high blood glucose level or a decreased kidney threshold concentration. When blood glucose levels exceed approximately 180 mg/dL, the proximal tubules become overwhelmed and cannot reabsorb the excess glucose. As a result, glucose is then excreted in the urine. [2] Additionally, because urinary dipstick tests detect the presence of glucose only, the Clinitest and Benedict qualitative test should be used for patients with suspected inborn errors of metabolism. [1]
Ketones in the urine are abnormal. Ketones accumulate when carbohydrates are insufficient and the body must get its energy from fat metabolism. [2] Acetone, acetoacetic acid, and B-hydroxybutyric acid are the common ketones formed. Ketonuria may be seen with uncontrolled diabetes, diabetic ketoacidosis, severe exercise, starvation, vomiting, and pregnancy. [1]
Nitrite testing is sensitive, but not specific, in detecting UTIs. Normally no nitrites are detected in the urine. Urinary nitrates are converted to nitrites by bacteria in the urine. A positive nitrite result signifies that bacteria capable of this conversion (eg, Escherichia coli, Klebsiella, Proteus, Enterobacter, Citrobacter, Pseudomonas) are present in the urinary tract. However, some bacteria are not capable of converting nitrates to nitrites (eg, Staphylococcus, Streptococcus, Haemophilus), and these bacteria may still be present in the urinary tract despite a negative test result. Therefore, a positive test suggests a UTI (typically due to Enterobacteriaceae), but a negative test result does not rule out a UTI. [1]
WBCs contain an enzyme known as leukocyte esterase, which is released when WBCs undergo lysis. Normally, too few WBCs are present in the urine for the test to be positive. However, when the number of WBCs in the urine increases, the result becomes positive. [1] A positive leukocyte esterase test result indicates pyuria. Pyuria typically implies a UTI. Sterile pyuria is seen in analgesic nephropathy and UTIs due to organisms that do not grow by standard culture techniques (eg, Chlamydia, Mycobacterium tuberculosis, Ureaplasma urealyticum). [1]
Bilirubin should not be present in the urine. In obstructive hepatobiliary conditions and in certain liver diseases, such as hepatitis, conjugated (water-soluble) bilirubin is excreted in the urine. Often, this may occur prior to the development of clinical symptoms (ie. jaundice). [2]
Urinary bilirubin may be present in low amounts in the urine. Bilirubin excreted into the intestine is metabolized by bacteria and forms urobilinogen. Urobilinogen is reabsorbed via the portal circulation and a small amount is excreted in the urine. [1] Increased urobilirubin levels are associated with excessive hemolysis, liver parenchymal diseases, constipation, and intestinal bacterial overgrowth. Decreased urobilirubin levels are associated with obstructive biliary disease and severe cholestasis.
Normal urinary proteins values are less than 150 mg/d and are undetectable using urinary dipstick. The urinary dipstick only detects the presence of albumin and no other proteins. When urinary protein values exceed 300-500 mg/d, the dipstick test result becomes positive. Thus, it is a very specific, but not sensitive, test for proteinuria. This is especially important to note in patients with diabetes because the urine dipstick is insensitive for microalbuminuria. The urine should not be tested within 24 hours after a contrast study because contrast (many iodinated radiocontrast agents) may produce false-positive results. [1]
In addition, the measurement of the proteinuria on the dipstick depends on the urine concentration. For example, on a small volume of concentrated urine, the dipstick may overestimate the amount of proteinuria. Whereas, on a large volume of dilute urine, the dipstick may underestimate the amount of proteinuria. This is why dipstick protein results must be quantified, as follows:
Trace proteinuria - Approximately 10-30 mg/dL, as follows:
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1+ - Approximately 30 mg/dL
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2+ - Approximately 100 mg/dL
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3+ - Approximately 300 mg/dL
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4+ - 1000 mg/dL or more
Because the urinary dipstick protein results can be inaccurate for the above stated reasons, a more accurate test is the sulfosalicylic acid test (SSA). This test detects all proteins in the urine at any amounts, including albumin, globulin, and Bence Jones proteins. However, just as with the urinary dipstick, contrast can cause false-positive results. The test is performed by mixing 3 parts of 3% sulfosalicylic acid with one part urine supernatant. Then, assess the turbidity of the solution. [1]
Sulfosalicylic acid test results are as follows:
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0 – No turbidity (proteinuria, 0 mg/dL)
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Trace – Slight turbidity (proteinuria, 20 mg/dL)
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1+ - Print visible through specimen (proteinuria, 50 mg/dL)
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2+ - Print invisible (proteinuria, 200 mg/dL)
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3+ - Flocculation (proteinuria, 500 mg/dL)
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4+ - Dense precipitate (proteinuria, ≥1000 mg/dL)
Patients with significant or persistent proteinuria should undergo a quantitative measurement of protein excretion. This can be accomplished by performing a 24-hour urine collection or by calculating the total urine protein to creatinine ratio using a single random urine specimen. A 24-hour urine collection can be difficult for elderly patients or those with incontinence and is especially cumbersome in the outpatient setting. Hence, the urine protein to creatinine ratio is typically the preferred method and has been shown to have a good correlation with the 24-hour urine protein determination. [1]
In addition to detection and quantification, identifying the etiology of proteinuria is important. Proteinuria is typically classified as transient or persistent. Overall, proteinuria can be classified as transient or persistent. Transient proteinuria is often benign and self-limited . Persistent proteinuria is then further classified into glomerular, tubular, or overflow. [3]
Transient proteinuria due to transient changes in glomerular hemodynamics (increased excretion of urinary protein) may have the following etiologies:
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Congestive heart failure
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Fever
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Strenuous exercise
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Seizure disorders
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Stress
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Orthostatic proteinuria
Glomerular proteinuria due to disruption of filtration barrier (an increased filtration of albumin across the glomerular capillary wall) may have the following etiologies:
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Nephrotic syndrome (ie, diabetic nephropathy)
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Glomerulonephritis
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Orthostatic proteinuria (rather benign course, lesser degrees of proteinuria [typically < 2 g/d])
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Exercise-induced proteinuria (rather benign course, lesser degrees of proteinuria [typically < 2 g/d])
Tubular proteinuria due to defective reabsorptive capacities in the proximal tubules of freely filtered proteins, mostly low&molecular weight proteins such as immunoglobulin light chains (excretion of normally reabsorbed proteins), may be caused by tubulointerstitial diseases (ie, ATN, acute interstitial nephritis, Fanconi syndrome).
Overflow proteinuria due to overproduction of immunoglobulin light chains in multiple myeloma (amount produced exceeds maximum amount for reabsorption in the tubules) may have the following etiologies:
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Multiple myeloma
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Myoglobinuria
The dipstick test for blood detects the peroxidase activity of RBCs. [1] If more than 3 RBCs are present, then the urinary dipstick test result is positive for blood. However, the urine dipstick does not detect where the blood is coming from. A positive blood result on the urine dipstick can represent hematuria, hemoglobinuria, myoglobinuria, false-positive results, or contamination. False-positives may be seen with alkaline urine (pH >9), sem*n in the urine, and urine contaminated with oxidizing agents used to cleanse the perineum. In addition, remember that positive results can also represent contamination with blood from a nonurinary source, such as hemorrhoids or vagin*l bleeding. [2, 3]
In general, clear supernatant and red urinary sediment is due to hematuria. A red supernatant that is heme positive on dipstick testing is due to hemoglobinuria or myoglobinuria. A red supernatant that is heme negative on dipstick is due to beets, food dyes, porphyria, hydroxocobalamin, phenazopyridine, or various other medications. [3]
Additionally, if the urinary dipstick is positive for blood and urine microscopy is positive for RBCs, hematuria is confirmed. If the dipstick result is positive for blood but no RBCs are found in the urinary sediment when analyzed on urine microscopy, then that indicates myoglobinuria (caused by rhabdomyolysis or myoglobinuric renal failure) or hemoglobinuria (caused by infections such as Plasmodium falciparum or Clostridium welchii, transfusion-related reactions, or paroxysmal nocturnal hemoglobinuria).
Microscopic examination/urine sediment
WBCs, RBCs, epithelial cells, and, rarely, tumor cells are the cellular elements found in the urinary sediment.
The number of WBCs considered normal is typically 2-5 WBCs/hpf or less. A high number of WBCs indicates infection, inflammation, or contamination. [1] Typically, most of the WBCs found are neutrophils. Urinary eosinophils and lymphocytes may also be found and can been seen with a Wright stain of the sediment. If found, urinary eosinophils may help diagnose acute interstitial nephritis (AIN). Eosinophiluria is seen with AIN, but the absence of eosinophiluria does not rule out AIN. Urinary lymphocytes are often associated with tubulointerstitial diseases. [3]
Hematuria can be gross or microscopic. Gross hematuria is the presence of red/brown urine. As little as 1 mL of blood per liter of urine can produce a visible color change; therefore, gross hematuria does not automatically indicate a large amount of blood. [3] In addition, as previously discussed in above sections, the red/brown color change could be due to contamination from other sources of blood (menstruation/hemorrhoids) and can also be seen without the presence of any RBCs (certain medications, beets, myoglobinuria, or hemoglobinuria).
Normally, less than 2 RBCs/hpf are observed. Microscopic hematuria is defined as the presence of 3 RBCs/hpf or more in 2 of 3 urine samples. [1] Hematuria may also be transient or persistent. Transient hematuria in young patients is fairly common and is typically benign. However, in older patients (>50 y), hematuria, even when transient, can be serious and warrants a full workup for possible underlying malignancy.
Persistent hematuria should always warrant a full evaluation. The causes of hematuria are often categorized as renal versus extrarenal. If the cause is thought to be renal, it is further categorized into glomerular versus nonglomerular. The hallmark findings of hematuria of glomerular origin include red cell casts, proteinuria (>500 mg/d), and dysmorphic RBCs. [1] Hematuria of glomerular origin is also commonly described as "cola-colored." [3]
The first step in the evaluation of a patient with hematuria is a detailed history. This may provide the clinician with important diagnostic clues. For example, hematuria with acute onset flank pain radiating to the ipsilateral groin with nausea/vomiting suggests nephrolithiasis, whereas dysuria suggests a UTI or pyelonephritis, if fevers/chills are also present. A patient who notes a recent upper respiratory infection should be evaluated for possible postinfectious glomerulonephritis or immunoglobulin (Ig)A nephropathy.
The timing is also important. Hematuria at the start of urination may indicate a distal urethra origin; hematuria at the end of urination may indicate bladder neck/posterior urethra/prosthetic urethra origin; and hematuria throughout urination suggests upper urinary tract/upper bladder origin. A patient’s family history is also important to gather because hematuria may also be due to familial disorders (ie, polycystic kidney disease, Alport syndrome, sickle cell nephropathy, thin basem*nt membrane nephropathy, benign familial hematuria). [1]
Glomerular causes of hematuria are associated with the following:
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Proteinuria - >500 mg/d
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RBC casts
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Dysmorphic RBCs
The causes of glomerular-based hematuria include the following:
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Thin basem*nt membrane nephropathy (benign familial hematuria)
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Alport syndrome
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IgA nephropathy
Nonglomerular causes of hematuria are associated with the following:
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Proteinuria - >500 mg/d
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No RBC casts
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No dysmorphic RBCs
Causes of nonglomerular-based hematuria include the following:
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Tubulointerstitial nephritis
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Pyelonephritis
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Polycystic kidney disease
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Renal cell carcinoma
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Sickle cell disease or trait
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Renovascular disease (eg, atheroembolic renal disease, renal vein thrombosis, arteriovenous malformations, “nutcracker syndrome”)
Extrarenal-based hematuria may be caused by the following:
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Tumors/malignancies (prostate, ureteral, bladder)
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Stones (kidney, bladder)
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Benign prostatic hyperplasia
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Infections (pyelonephritis, cystitis, prostatitis, urethritis)
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Schistosomiasis
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Foley trauma
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Anticoagulants
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Chemotherapeutic agents (mitotane, ifosfamide, cyclophosphamide)
Epithelial cells that may be found in the urinary sediment include squamous epithelial cells (from the external urethra) and transitional epithelial cells (from the bladder). [2] Generally 15-20 squamous epithelial cells/hpf or more indicates that the urinary specimen is contaminated. [1]
Hyaline casts may be seen in healthy individuals. Other types of casts are not normally found and are suggestive of renal disease. In particular, the finding of cells within a cast is diagnostic of an intrarenal origin. [3] Casts are cylindrical particles that are formed from coagulated protein secreted by tubular cells. The organic matrix is mainly composed of Tamm-Horsfall mucoprotein (which glues or cements casts together). They are usually cylindrical with regular margins, as they are formed in the long, thin, hollow renal tubules and typically take the shape of the tubule in which they are formed. They are only formed in the distal convoluted tubule or the collecting duct and not in the proximal convoluted tubule or in the loop of Henle. Low urine pH, low urine flow rate, and high urinary salt concentration promote cast formation (by favoring protein denaturation and precipitation). [1]
Hyaline casts are found in healthy individuals and are relatively nonspecific. They may be increased after strenuous exercise. They are often seen in small volumes of concentrated urine or with diuretic therapy. [3] Red cell casts are nearly diagnostic of glomerulonephritis or vasculitis. White cell casts and pyuria are most commonly seen with tubulointerstitial nephritis and acute pyelonephritis. WBC casts are also seen with renal tuberculosis and vagin*l infections. "Muddy-brown" granular casts are diagnostic of acute tubular necrosis. Waxy and broad casts are consistent with advanced renal failure. Fatty casts and lipiduria, with the typical "maltese-cross" appearance on polarized microscopy, are commonly seen with nephrotic syndrome. [1]
Crystals are solid forms of a particular dissolved substance in the urine. Identifying factors of crystals include shape, color, and urine pH. [2] Crystal formation is determined by the urine pH, the supersaturation of the molecules, and the presence of possible inhibiting factors. [1] Crystalluria may be normal when the crystals are composed of solutes that are usually found in the urine. However, the observation of certain urinary crystals can diagnostically significant. For example, calcium oxalate crystals ("envelope-shaped") and acute kidney injury is seen with ethylene glycol ingestion. The presence of large amounts of uric acid crystals ("diamond" or "barrel" shaped) and acute kidney injury is seen in tumor lysis syndrome. Uric acid crystals may also be seen with other causes of hyperuricosuria, such as gout. In addition, cystine crystals ("hexagonal")areseen with cystinuria. Finally, magnesium ammonium phosphate and triple phosphate crystals (struvite) are "coffin-lid" shaped and seen with UTIs caused by urea-splitting organisms (ie, Proteus, Klebsiella). [1]
Bacteria in the urine sediment are generally due to infection or contamination. Normally no bacteria should be seen in the urinary sediment. However, given the abundance of normal microbial flora in the vagin* and/or external urethral meatus, this is not an unusual finding. In addition, bacteria multiply rapidly if the urine specimen is left standing for too long in room temperature. A urinalysis with positive tests for nitrites, leukocyte esterase, and bacteria is highly suggestive of a urinary tract infection. However, if a significant amount of squamous epithelial cells (≥15-20/hpf) are present as well, these findings may primarily indicate a contaminated specimen and the urinalysis should be repeated. Although, even if no squamous cells are present and true bacteriuria is found, these findings should be correlated clinically with the presence of symptoms consistent with a urinary tract infection. [1] If the patient does not have concomitant symptoms consistent with a UTI, then it istermedasymptomatic bacteriuria. Except for in certain circ*mstances, asymptomatic bacteriuria is generally not treated.
If bacteriuria is found and a UTI is suspected, a urine culture with sensitivities is recommended. In catheterized patients or with urine obtained from a suprapubic tab, any organism on the urine culture is considered significant. Otherwise, generally, 100,000/mL or more of a single organism reflects significant bacteriuria. The presence of multiple organisms, especially at less than 100,000/mL, typically suggests polymicrobial contamination. [1]
Yeast cells are not normally found in the urine specimen. They can be distinguished from red cells and amorphous crystals by their tendency to bud. Commonly the yeast cells are of the Candida species, which can colonize the vagin*, urethra, or bladder. Yeast cells may signify true infection or contamination (often due to contamination by vagin*l secretions in women with a yeast infection). [1]
