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Urinalysis & Body Fluids

Chemical

This is the heart of the urinalysis, where we move from simple observation to chemical analysis. The reagent strip is our rapid screening tool, and the confirmatory tests are our specialized methods for verifying critical results

Chemical Examination of Urine

The chemical examination is a crucial screening portion of the routine urinalysis. It uses a multi-pad reagent strip to rapidly and semi-quantitatively assess a panel of chemical markers in the urine. Think of it as the “vital signs” of the urine—it provides a snapshot of metabolic function, renal health, and potential infections. However, the reagent strip is only a screening tool. Positive or unexpected results must often be verified using more specific and sensitive confirmatory tests. The interplay between the strip and these follow-up tests is central to an accurate chemical workup

The Reagent Strip: The Workhorse of Urinalysis

The reagent strip is a marvel of efficiency. This simple plastic strip is embedded with up to ten different chemically-impregnated pads, each designed to react with a specific analyte in the urine and produce a color change. The accuracy of these results is entirely dependent on proper technique, which includes: * Using a fresh, well-mixed, uncentrifuged specimen * Dipping the strip briefly to saturate all pads * Blotting the edge to remove excess urine and prevent “run-over” between pads * Reading the color changes at the precise times specified by the manufacturer

The tests on a standard strip can be grouped by their primary clinical purpose:

Markers of Renal Health and Hydration

  • Protein: The single most important indicator of renal disease. The strip test is based on the “protein error of indicators” principle and is most sensitive to albumin. A positive result suggests glomerular damage
  • pH: Measures the acidity or alkalinity of the urine. It provides clues about acid-base disorders, diet, and the presence of urea-splitting bacteria, and is essential for interpreting crystal formation
  • Specific Gravity: An indirect measure of urine concentration based on the ionic content of the specimen. It reflects the patient’s hydration status and the kidney’s concentrating ability

Markers of Metabolic Function

  • Glucose: A specific enzymatic test that screens for glucose in the urine (glucosuria). Its presence indicates that the blood glucose level has exceeded the kidney’s renal threshold (160-180 mg/dL), a hallmark of Diabetes Mellitus
  • Ketones: Detects the products of fat metabolism (ketonuria). Ketones appear when the body cannot use carbohydrates for energy, a key finding in diabetic ketoacidosis (DKA), starvation, or low-carb diets

Markers of Liver Function and Hemolysis

  • Bilirubin: Detects conjugated bilirubin in the urine. It is an early indicator of liver disease (like hepatitis) or biliary tract obstruction. Bilirubin is very unstable and breaks down in light
  • Urobilinogen: A small amount is normal. An increased result can indicate liver disease or hemolytic anemia. An absent result is very significant, suggesting a biliary obstruction
  • Blood: Detects the pseudoperoxidase activity of heme, meaning it reacts positively with intact RBCs (hematuria), free hemoglobin (hemoglobinuria), and myoglobin (myoglobinuria)

Markers of Infection and Inflammation

  • Nitrite: A highly specific indicator for the presence of nitrate-reducing bacteria (most common UTI pathogens). A positive result is a strong predictor of a UTI
  • Leukocyte Esterase (LE): Detects an enzyme released from WBCs. A positive result indicates pyuria (WBCs in urine) and is a key sign of inflammation, most often due to a UTI

Confirmatory Tests: Verifying the Results

When a reagent strip result is unexpected, ambiguous, or critically important, we turn to confirmatory tests. These tests typically use a different chemical principle, making them less subject to the same interferences as the strip test, and are often more sensitive or specific

  • Protein → Sulfosalicylic Acid (SSA) Precipitation Test
    • Why it’s used: The reagent strip primarily detects albumin. The SSA test is a precipitation method that will detect all proteins, including globulins and the Bence-Jones protein associated with Multiple Myeloma, which the strip can miss. It is used to confirm any positive protein result and to screen for non-albumin proteins in suspicious cases
  • Ketones → Acetest Tablets
    • Why it’s used: Acetest tablets use the same sodium nitroprusside reaction as the strip but are generally more sensitive. They can be used to confirm a questionable low-positive result on the strip or to test for ketones in other body fluids like serum or plasma
  • Bilirubin → Ictotest Tablets
    • Why it’s used: The Ictotest is significantly more sensitive than the reagent strip for bilirubin. Because bilirubin can be a very early indicator of liver disease and is unstable in light, a confirmatory test is often necessary to verify a faint positive or to rule out a false negative in a suspicious sample (e.g., dark amber urine)
  • Glucose → Clinitest (Copper Reduction Test)
    • Why it’s used: This is a classic test with a specific, modern application. The reagent strip is specific for glucose. The Clinitest is a non-specific copper reduction method that detects all reducing sugars, including glucose, galactose, fructose, etc. Its primary clinical utility today is in screening newborns and infants for inborn errors of carbohydrate metabolism, most importantly galactosemia, where the enzyme to break down galactose is missing. A baby with a negative glucose strip but a positive Clinitest could have a life-threatening metabolic disorder