Section 17 Nephrology Chapter 127 Urinalysis in Clinical Practice Sekhar Chakraborty INTRODUCTION TABLE 1 │ Routine urinalysis Examination of urine is an indispensable part of evaluation of Appearance patients with impaired kidney function, particularly proteinuria, Specific gravity hematuria, urinary tract infection, nephrolithiasis and other renal or Chemical tests (Dipstick) nonrenal diseases. The relatively simple chemical test performed in • pH the routine urinalysis rapidly provides important information about • Protein primary kidney disorder and systemic diseases. Examination of urine • Glucose sediment provides valuable information about renal parenchyma. • Ketones Dipstick test can be automated and there is no substitute for careful • Blood examination of urine under the microscope. Experience in examining the urine is valuable; studies show that a urinalysis performed by an • Urobilinogen experienced nephrologist or trained physician is more likely to be • Bilirubin better than a urinalysis reported by a clinical chemistry laboratory. • Nitrites • Leukocyte esterase SPECIMEN COLLECTION AND HANDLING • Microscopic examination (formed element). Crystals Urate, calcium phosphate, oxalate or carbonate, Urine should be collected with minimum contamination. triple phosphate, cysteine, drugs • Clean catch midstream collection is performed Cells Leukocytes, erythrocytes, renal tubular cells, oval • If not feasible, bladder catheterization is appropriate for adults— fat bodies, transitional epithelium, squamous cell risk of contracting a urinary tract infection is negligible for a Casts Hyaline, granular, red blood cell (RBC), white single catheterization blood cell (WBC), tubular cell, degenerating • Suprapubic aspiration is used in infants cellular, broad, waxy, lipid-laden • High urine osmolality and low pH favor cellular preservation, Infecting Bacteria, yeast, trichomonas, nematodes hence first voided morning urine is preferred organism • Chemical composition of urine changes with standing and formed Miscellaneous Spermatozoa, mucous threads, fiber, starch, hair elements degrade over time. Hence, urine is best examined when and other contaminants fresh but a brief period of refrigeration is acceptable • Bacteria in urine multiply at room temperature, hence bacterial counts from unrefrigerated urine are unreliable (Table 1). Specific gravity near 1.010 connotes isosthenuria, with a urine osmolality same that of plasma. ROUTINE URINALYSIS Methods for Measurement of Specific Gravity Appearance • By hydrometer: Sufficient volume of urine required. Selected substance that may alter the physical appearance or odor of • By refractometer: Calibrated in specific gravity units, a drop of the urine has been shown in Table 2. urine is required. • By dipstick: Small amount of urine is required. Specific Gravity Clinical Importance of Measuring Specific Gravity Weight of urine of measured volume Specific gravity =_____________________________________ • In the absence of proteinuria, glycosuria or iodinated contrast Weight of distilled water of some volume administration, a specific gravity more than 1.018 implies Urine specific gravity is inaccurate surrogate for osmolality. preserved concentrating ability of kidneys. Urinary specific gravity is usually between 1.001 and 1.035 and • Measurement is useful to differentiate prerenal azotemia and urine osmolality is usually between 50–1,000 mOsm/kg. acute tubulointerstitial nephritis (ATIN). Section 17 Chapter 127 Urinalysis in Clinical Practice TABLE 2 │ Selected substance that may alter the physical appearance or odor of the urine Color change Substance White Chyle, pus, phosphate crystals Pink/red/brown Erythrocytes, hemoglobin, myoglobin, porphyrins, beets, senna, cascara, levodopa, methyldopa, deferoxamine Phenolphthalein and congeners, food colorings, metronidazole, phenacetin, anthraquinones, doxorubicin Phenothiazines Yellow/orange/brown Bilirubin, urobilin, phenazopyridine urinary analgesics, senna, cascara, mepacrine, iron compounds, nitrofurantoin, riboflavin, rhubarb, sulfasalazine, rifampin, fluorescein, phenytoin, metronidazole Brown/black Methemoglobin, homogentisic acid (alkaptonuria), melanin (melanoma), levodopa, methyldopa Blue/green, green/brown Biliverdin, pseudomonas infection, dyes (methylene blue and indigo carmine), triamterene, vitamin B complex, methocarbamol, indicant, phenol, chlorophyll, propofol, amitriptyline, triamterene Purple Infection with Escherichia coli, pseudomonas, enterococcus, others Odor Substance or condition Sweet or fruity Ketones Ammoniacal Urea splitting bacterial infection Maple syrup Maple syrup urine disease Musty or mousy Phenylketonuria “Sweet feet” Isovaleric or glutamic acidemia or excess butyric or hexanoic acid Rancid Hypermethioninemia, tyrosinemia Chemical Composition of Urine TABLE 3 │ Protein composition of normal urine Routine Dipstick Methodology Plasma proteins Excretion (mg/day) Albumin 12 Proper tabs impregnated with chemical reagents are fixed to a plastic Immunoglobulin G 3 strip. Reagents are chromogenic altered with a chest, which is highly specific. Some interfering substance in urine or extremes of pH may Immunoglobulin A 1 alter the results. Immunoglobulin M 0.3 Physiologic urinary pH lies between 4.5 and 8. Light chains pH should be tested properly in freshly voided urine because: κ 2.3 λ 1.4 • Growth of urea splitting bacteria and loss of carbondioxide (CO2) raise the pH β-microglobulins 0.12 • Bacterial metabolism of glucose may produce organic acids and Other plasma proteins 20 lowers the pH All plasma proteins 40 • These strips are not sufficiently accurate to be used for the Nonplasma proteins diagnosis of renal tubular acidosis (RTA). Tamm-Horsfall protein 40 Protein Dipstick Method Other nonrenal derived proteins < 1 All nonplasma proteins 40 Protein indicator strips are buffered at an acid pH near their color Total proteins 80 ± 24 (SD) change point; wetting them with a protein containing specimen indicates a color change. Abbreviation: SD, Standard deviation Protein reaction may be scored as follows: Source: Glasscock RJ. Proteinuria. In: Massry SG, Glassock RJ (Eds). Trace = 5–20 mg/dL Textbook of Nephrology, 3rd edition. Philadelphia: Williams & Wilkins; 1995 1+ = 30 mg/dL 2+ = 100 mg/dL 3+ = 300 mg/dL Proteinuria is evaluated on the basis of their origin and types, 4+ = > 2,000 mg/dL which is further investigated in Table 5 and represented in Flow Protein strips are highly sensitive to albumin but less so to chart 1. globulins, hemoglobin or light chains. CHEMICAL COMPOSITION OF THE URINE Acid Precipitation Test: Turbidimetric Method Routine Dipstick Methodology • Urine that is negative by dipstick but positive with sulphosalicylic The urine dipstick is a plastic strip to which paper tabs impregnated acid is highly suspicious for light chains. with chemical reagents have been affixed. The reagents in each • Tolbutamide, high-dose penicillin, sulfonic acid and radiographic- are chromogenic. After timed development, the color on the paper contrast agents may yield false-positive turbidimetric reactions. segment is compared with a chart. Some reactions are highly specific. • Protein indicator used for routine dipstick analysis is not sensitive Others are sensitive to the presence of interfering substance or enough to detect microalbuminuria. extremes of pH. Discoloration of the urine bilirubin or blood may obscure the color. Protein Composition of Urine pH Protein composition of normal urine is represented in Table 3. Causes of proteinuria according to pathophysiology are shown in The pH test pads use indicator days that change color with pH. The Table 4. physiologic urine pH ranges from 4.5 to 8. The determination is most 577 Nephrology Section 17 TABLE 4 │ Causes of proteinuria according to pathophysiology Protein Glomerular proteinuria Protein measurement uses the protein error of indicators principle. • Primary glomerular disease The pH at which some indicators change color varies with the protein – Minimal change glomerulopathy concentration of the bathing solution. Protein indicator strips are – Immunoglobulin A nephropathy buffered at an acid pH near their color change point. Wetting them – Focal segmental glomerulosclerosis (FSGS) – Membranous glomerulonephritis with a protein containing specimen induces a color change. The – Membranoproliferative glomerulonephritis (MPGN) protein reaction may be scored from trace to 4+ or by concentration. – Fibrillary and immunotactoid glomerulopathy Highly alkaline urine, especially after contamination with quaternary – Crescentic glomerulonephritis ammonium skin cleansers, may produce false-positive reactions. • Secondary glomerular disease Protein strips are highly sensitive to albumin but less so to – Multisystem disease: Systemic lupus erythematosus (SLE), globulins, hemoglobin or light chain. If light-chain proteinuria vasculitis, amyloid, scleroderma is suspected, more sensitive assays should be used. With acid – Metabolic disease: Diabetes mellitus, Fabry’s disease precipitation test, an acid that denatures protein is added to the – Neoplasia: Myeloma, leukemia, solid tumors – Infections: Bacterial, fungal, viral, parasitic urine specimen and the density of the precipitate is related to – Drugs, toxins and allergens: Gold, penicillamine, lithium, the protein concentration. Urine that is negative by dipstick but nonsteroidal anti-inflammatory drug (NSAID), penicillin positive with sulfosalicylic acid is highly suspicious for light chains. – Familial: