Urinalysis and diagnostic tests





Urinalysis


Urine examination is an invaluable tool in patient evaluation and diagnosis. Easy to obtain but often overlooked, the voided urine offers clues to the diagnosis of kidney conditions and helps with the prognosis in cardiovascular disease and diabetes.


A complete urinalysis consists of visual inspection for color and turbidity, a dipstick exam, and a microscopic exam of the centrifuged urinary sediment. While the dipstick is a “waived” test that can be done in any office, the microscopic exam requires Clinical Laboratory Improvement Amendments (CLIA) certification, so it is usually performed by a clinical laboratory, nephrologist, or trained physician or technician.


6.1 Urine color


























































































































































Color Causes
Cloudy or turbid Pyuria Heavy crystalluria
Bacteriuria Fecal or vaginal contamination
White WBCs Chyle
Red or cola-colored RBCs (supernatant may be clear)
Cascara
Free Hgb Senna
Myoglobin Beets
Porphyria (porphobilin porphyrins)
Dye Drugs
Phenolphthalein Doxorubicin
Phenazopyridine
Phenytoin
Yellow Vitamin supplement Dye
Riboflavin Yellow dye
Dark yellow/orange Drugs
Sulfasalazine
Rifampin
Bilirubin
Phenazopyrazine
Black/brown Drugs Other
Methyldopa Melanin
Levodopa Condition
Metronidazole Homogentisic acid (alkaptonuria)
Imipenem-cilastatin
Dull blue/green Drugs Drug/dye
Triamterene Methylene blue
Amitriptyline Infection
Propofol Pseudomonas UTI
Endogenous metabolite: Biliverdin
Purple Seen in the setting of foley bag collection, not in the passed urine
Infection
Proteus mirabilis
Escherichia coli
Pseudomonas aeruginosa
Morganella morganii
Klebsiella pneumoniae
Enterococcus spp.

Hgb , Hemoglobin; RBCs , red blood cells; UTI , urinary tract infection; WBCs , white blood cells.


6.2 Dipstick test















































Dipstick Positive False Positive False Negative
Blood Detects RBCs as low as 1–2/hpf, but also reacts to free Hgb (hemolysis) and myoglobin (rhabdomyolysis) Uncommon contamination with hypochlorite or bacteria with pseudoperoxidase activity Rare; high concentration of ascorbic acid can mask low-grade hematuria
Proteins Detects albumin >30 mg/dL (normal is negative; trace- 27% will have microalbuminuria; 1+protein- 47% will have micro- or macroalbuminuria) Highly buffered alkaline urine, some antiseptics, such as chlorhexidine, ejaculation Light chains and other immunoglobulins, beta-2-microglobulin, dilute urine with <30 mg/dL of albumin
Nitrites Denotes Enterobacteriaceae, which convert urinary nitrate to nitrite in UTI None Short incubation time in the bladder (<4 hr), pathogen doesn’t convert nitrate to nitrite, not enough nitrate or too much ascorbic acid in urine
Glucose Detects diabetes and renal glucosuria None Ascorbic acid or ketones in urine decrease positive tests
Ketones Detects acetoacetate in diabetic and starvation/alcoholic ketosis High quantities of levodopa, mesna, tiopronin, captopril, penicillamine, acetylcysteine in urine Doesn’t detect acetone or beta-hydroxybutyrate ketone bodies
Leukocyte esterase Detects pyuria ≥6 WBCs/hpf but may indicate interstitial nephritis or leukemia rather than infection Contamination with vaginal discharge or saliva High glucose or specific gravity, some antibiotics, only mononuclear leukocytes in transplant rejection
SG Measures urine concentration, correlating roughly with osmolarity, normally SG = 1.003–1.030 (see figure below) High SG >1.030 usually due to radiocontrast; also, glucose or protein will raise SG > osmolarity Low SG shows dilute urine, but agents affecting urine color cause inaccurate results

Hgb , Hemoglobin; hpf , high power field; RBCs , red blood cells; SG , specific gravity; UTI , urinary tract infection.


6.3 Proteinuria


6.3.1 Protein to creatinine ratio and albumin to creatinine ratio


6.3.1.1 Estimating the degree of proteinuria in a random or spot urine sample ,


Urine protein to creatinine ratio (PCR) measures all proteins, can be expressed as grams of protein per gram of creatinine (Cr) or a unit-free number, and is calculated as:



Uprotein(mg/dL)Ucreatinine(mg/dL)=protein(g)creatinine(g)


This correlates with grams of protein excreted per day per 1.73 m 2 body surface area (BSA).


6.3.1.2 Detecting micro- or macroalbuminuria (see below for definitions) in a random or spot urine sample


Urine albumin to creatinine ratio (ACR) is expressed as milligrams of albumin per gram of Cr and is calculated as:



Ualbumin(mg/L)Ucreatinine(g/L)=albumin(mg)creatinine(g)

























Proteinuria PCR on Spot Urine 24-Hour Urine Collection
Normal <0.2 (usually unit-free but can be g/g) <150 mg/24 hr
Mild proteinuria 0.2–1.0 150 mg–1.0 g/24 hr
Moderate proteinuria 1.0–3.0 1.0–3.0 g/24 hr
Nephrotic range >3.0 >3.0 g/24 hr





















Albuminuria ACR on Spot Urine 24-Hour Urine Collection
Normal


  • <30 mg/g is considered normal, but sex-based normals are:




    • <17 mg/g (males)



    • <25 mg/g (females)


<30 mg/24 hr
Microalbuminuria


  • 30–300 mg/g (either sex)



  • or 17–300 mg/g (males)



  • 25–300 mg/g (females)

30–300 mg/24 hr
Macroalbuminuria >300 mg/g >300 mg/24 hr





6.4 Estimation of renal function


6.4.1.1 Measured creatinine clearance (CrCl)


Collect timed urine for volume and total Cr excretion (usually 24-hour collection in which total urine volume is 1440 min/24 hr in the formula below):



Normal daily Cr excretion: Males: 15−20 mg/kg/24 hr*Females: 10−15 mg/kg/24 hr*(*assuming normal muscle mass)Creatinine clearance mL/min=UCr(mg/dL)×UVol(mL/min)SCr(mg/dL)


6.4.1.2 Measured glomerular filtration rate (GFR)


Rarely done except for research studies:



  • ■

    Inulin clearance


  • ■

    I 125 iothalamate clearance



Estimated GFR using cystatin C :



  • ■

    Biomarker similar to Cr used to estimate GFR


  • ■

    Is filtered but not secreted


  • ■

    Avoids confounding factors seen with Cr related to diet and muscle mass


  • ■

    Typically is a test that is sent out, so results take longer than they do for Cr



6.4.1.3 Cockcroft-gault formula


Estimated creatinine clearance calculated from serum creatinine (SCr):


eCCr(ml/min)=(140−age)SCr(mg/dL)×Body weight(kg)72×(0.85 if female)


Original formula used actual body weight (BW), but to avoid overestimate of eC Cr if weight is >120% of ideal body weight (IBW), it is better to use:



Either IBM or0.4(BW−IBW)+IBW where IBW is calculated as:• IBW for females(kg)=45+(2.3×inches over 60″)IBW for males(kg)=50+(2.3×inches over 60″)

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Sep 9, 2023 | Posted by in NEPHROLOGY | Comments Off on Urinalysis and diagnostic tests

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