Chemical Analysis of Urine - Part 1 Notes

Overview

  • Urine chemical analysis (part 1) is a quick, easy screening tool used to rule in/out disease, monitor therapy, and track disease progression.
  • Focus on constituents in urine that are not visible to the naked eye.
  • We use reagent strips (dipsticks) that have multiple pads for different analytes.
  • Common dipstick reagents on urine strips: pH, white blood cells (WBCs), blood, protein, glucose, and nitrites (bacteria). Some slides include additional pads; others are single-analyte tests (e.g., microalbumin, myoglobin).
  • Reagent strips provide rapid results and are used for screening and monitoring; results can be read visually or by automated readers.
  • Interfering substances can affect results; the most common is ascorbic acid (vitamin C). Some pads may include a pad to detect ascorbic acid to alert the operator.
  • In addition to dipsticks, tablet tests exist for ketones, bilirubin, and reducing substances. They are used to confirm dipstick results or when dipstick readings are unreliable (e.g., highly pigmented urine).
  • Clinitest (reducing substances) is specifically used for patients 2 years and younger.
  • Other chemical tests include pH meters, pH test papers, sulfosalicylic acid (SSA) for protein, and porphobilinogen (PBG) for urobilinogen. These last two are not commonly performed in all labs.
  • Always follow the manufacturer’s instructions for reagent strips and tablets. Keep strips away from light, moisture, and contaminants. Do not use strips past their expiration date.
  • QC (quality control) is essential for urinalysis: typically two levels per shift (normal and abnormal).
  • Sample labeling is critical; mislabeling is a common reason for rejecting a urine sample. A typical routine urinalysis requires at least 10–12 mL of urine. If urine is >2 hours old, refrigerate for preservation; warm to room temperature before testing and mix well.
  • The testing workflow includes using a pour-over tube, dipping the strip briefly, saturating pads, removing excess, and reading at the specified times in good lighting, with the color chart handy. Some labs use automated readers to interpret the pad colors.
  • Correlate chemical results with the physical appearance of urine and microscopic findings (e.g., blood on dipstick should correlate with urine color/appearance).
  • Age-specific considerations: certain tablet tests are used for specific age groups (e.g., Clinitest for patients ≤2 years).
  • Pyridium (phenazopyridine) is a drug that can cause bright orange/red urine and can interfere with many dipstick tests; always review patient medication history.
  • The lecture will continue with leukocyte esterase in the next session.

Reagent strips (dipsticks): composition and purpose

  • Reagent strips are long strips with pads for different analytes; common analytes include:
    • Urine pH
    • White blood cells (WBCs)
    • Blood (heme peroxidase activity)
    • Protein
    • Glucose
    • Nitrites (bacteria indicator)
  • Some pads detect ascorbic acid (to flag interference) or have overlays to minimize interference from substances like blood.
  • Some dipsticks are single-analyte tests, such as microalbumin (detects very small amounts of albumin) and myoglobin (identifies muscle breakdown).
  • Microalbumin is an early indicator of renal disease; albumin is a protein, and its appearance in urine indicates possible kidney damage.
  • Myoglobin indicates muscle breakdown and is detected specifically to differentiate it from hemoglobin (see later for how to distinguish).
  • The visual readout on the strip shows color changes for each analyte; NOA (analyte name) is listed for each pad and changes are interpreted against a color chart.

Reagent strip handling, storage, and validity

  • The reagent pad vial is dark in color; pads must be protected from light to avoid color changes and inaccurate results.
  • Keep strips away from moisture and other chemicals.
  • Do not use reagent pads past their expiration date.
  • The typical setup includes a urine cup and a pour-over tube for dipping the strip.
  • For best results, read results at the specified times and in a well-lit area; keep the strip vial or a color chart nearby for reference.

Tablet tests: ketones, bilirubin, and reducing substances

  • Ketone tablet test (acid test) is used to confirm ketone presence detected on the dipstick.
  • Bilirubin tablet test (Ictotest/Ichotest) is used to confirm bilirubin when the dipstick is positive or when color interference makes reading difficult; tablets are more sensitive.
  • Reducing substances tablet test (Clinitest) is used as an additional test, especially when dipstick results are inconclusive or when pigments in urine interfere with dipstick readings.
  • Practical rules:
    • A positive ketone on the dipstick is confirmed with the ketone tablet test.
    • A positive bilirubin on the dipstick is confirmed with the Ictotest tablet.
    • Tablet tests are more sensitive and can be used as confirmatory tests.
  • Clinitest is specifically used for patients 2 years and younger.
  • Tablet tests may be used when urine color is too pigmented for reliable dipstick readings.

Other chemical testing methods

  • pH meter or pH test paper: used to determine urine pH in certain clinical scenarios;
    • pH can be tested directly with meters or test papers as a verification method for the dipstick results.
    • pH meters are temperature dependent and require calibration with three known buffers.
  • Sulfosalicylic acid (SSA) test: used to confirm protein in urine.
  • Porphobilinogen (PBG) test: assesses urobilinogen; not commonly performed in many laboratories.
  • Noted that these last two (SSA and PBG) are not routinely performed in all labs, but may be used in specific cases.

Quality control (QC) and sample handling in urinalysis

  • For urine QC, laboratories typically run two levels: a normal control and an abnormal control.
    • Normal control can be pure water, since urine normally lacks the analytes discussed.
    • Abnormal control is purchased from a manufacturer and contains positive controls for multiple analytes.
  • Tablet tests and other chemical tests have QC requirements as per lab protocol; in some settings, QC may not be performed every shift if those tests are not run every shift.
  • When a urine sample is received in the lab:
    • It must be properly labeled to avoid misidentification and misplacement.
    • A routine urinalysis typically requires at least 10-12\,\text{mL} of urine.
    • If the urine is fresh, proceed; if it is more than 2\,\text{hours} old, preserve in the refrigerator.
    • Before testing, mix the urine thoroughly to resuspend any sediment.
    • If urine has been refrigerated, allow it to reach room temperature before testing.

Urine collection, labeling, and preparation for testing

  • Proper labeling is essential because mislabeled urine is a common reason for sample rejection.
  • Volume considerations: ensure at least 10\text{–}12\,\text{mL} for routine urinalysis.
  • Freshness: test within 2\text{ hours} if possible; if not, refrigerate promptly.
  • If testing refrigerated urine, warm to room temperature before testing to avoid temperature-related artifacts.
  • Before dipping the strip, swirl the urine to mix any solids that may have settled.

Testing procedure: dip, read, and record

  • Procedure for dipstrip testing:
    • Use the pour-over tube to immerse the dipstick briefly into the urine.
    • Remove the dipstick and ensure all pads are thoroughly saturated for a uniform color development.
    • Tap off excess urine by brushing it against the side of the container onto an absorbent pad.
    • Read each test at the appropriate time using a color chart or printed NOA (analyte) chart provided by the manufacturer.
    • Read in a well-lit area; have the reagent strip vial or a color chart handy for reference.
  • Reading method:
    • Some labs rely on automated readers to interpret pad colors; others rely on visual inspection.
    • If a test is positive, correlate with the physical urine findings (e.g., a positive blood pad with visibly red urine strengthens the result).
  • Special handling for certain samples:
    • For babies or young children, follow age-specific testing guidelines (e.g., Clinitest for those ≤2 years).
    • If urine color is too pigmented, rely on tablet tests for confirmation.
  • Correlation to other data:
    • Always correlate dipstick results with the patient’s urine color and, when available, microscopic findings and patient history (e.g., recent menstruation, catheterization).

Reporting and interpretation of results

  • Reporting formats for common analytes vary by manufacturer and lab protocol; the report may include:
    • Glucose: reported as concentration (e.g., ext{mg/dL}) or as descriptive levels (e.g., small/moderate/large).
    • Red blood cells (RBCs) or white blood cells (WBCs): reported using a semi-quantitative scale (e.g., negative, trace, small, moderate, large) or as 1+, 2+, etc.
    • Nitrites: reported as positive or negative; nitrites indicate possible bacterial infection.
  • It is not required to memorize exact reporting conventions for every NOA; operators should follow the manufacturer's insert and lab protocol.

Interferences, pigments, and special cases

  • Ascorbic acid (vitamin C) is a universal interference: it is a reducing agent and can cause false negatives in several tests, including the blood/hemoglobin pad.
  • Strong oxidizing agents can cause false positives on dipstick tests.
  • Highly pigmented urine (e.g., from medications like pyridium) can obscure color changes on dipsticks and affect results. Pyridium causes bright orange/red urine.
  • Menstrual contamination or hemorrhoids can cause false positives on certain tests.
  • Some manufacturers provide an additional pad or overlay to detect ascorbic acid or to reduce interference from common substances (e.g., blood).
  • Distinguishing myoglobin vs hemoglobin in urine:
    • Myoglobin (from muscle breakdown) may appear on the dipstick; hemoglobin (from red blood cells or hemolysis) may also appear.
    • To differentiate, assess the patient’s plasma:
    • If plasma shows hemolysis, hemoglobin is likely present in urine.
    • If plasma is normal with no hemolysis, myoglobin may be present in urine.
  • Myoglobin in urine is associated with muscle injury/damage (trauma, heavy exertion, alcohol overdose).
  • Blood in urine (hematuria) can be due to normal physiological factors (e.g., menstruation) or procedures (catheterization), so correlate with history and microscopic exam.
  • The dipstick primarily detects the heme moiety (pseudoperoxidase activity); intact red blood cells can produce a dotted pattern on the pad when present.
  • Visual correlation with microscopic exam is essential for accurate interpretation of hematuria/hemoglobinuria/myoglobin.

Specific analytes on the dipstick (high-level details)

  • Specific gravity (indirect measurement of urine concentration):

    • Normal range for dipstick SG: 1.002 ext{ to } 1.030; physiologically impossible to be below 1.002 or above 1.04.
    • The SG pad responds to ionic solutes; when dipstick is immersed, protons are released from a poly electrode proportional to ionic concentration, changing pad color.
    • Result is an estimate, reported in increments of 0.005 (i.e., 1.002, 1.007, 1.012, 1.017, 1.022, 1.027, 1.032, with practical limits around 1.030).
    • Interferences: elevated glucose can give an abnormally low SG; elevated protein, lactic acid, or ketones can give an abnormally high SG.

  • Urine pH:

    • Normal range: 4.5
      leq ext{pH}
      leq 8.0.
    • After eating, urine tends to be more alkaline (alkaline tide).
    • It is physiologically impossible to have pH outside the [4.5, 8.0] range; values outside require clinical investigation.
    • The dipstick uses a double indicator system for pH.
    • Interference: Generally no interference for pH with storage issues; improper storage (e.g., left out >2 hours) can alter pH.
    • Reading increments: often reported in 0.5 or 1.0 unit steps (e.g., pH = 5.0, 5.5, 6.0).
    • Alternatives: pH meter or pH test paper may be used to confirm/verify dipstick results.
    • pH meter notes: temperature dependent; must calibrate with three known buffers (e.g., pH = 4.0, 7.0, 9.0 or similar acidic/neutral/alkaline references).
    • pH test paper usage can extend to other specimens (e.g., semen analysis).

  • Blood (hematuria/hemoglobinuria/hemoglobin/myoglobin):

    • Hematuria = red blood cells in urine (intact RBCs).
    • Hemoglobinuria = free hemoglobin in urine (often from hemolysis); may have clear urine and absence of intact RBCs on microscopy.
    • Dipstick detects both intact RBCs and hemoglobin through heme peroxidase activity; it can also detect myoglobin due to the heme moiety.
    • Distinguishing myoglobin vs hemoglobin:
    • If plasma shows hemolysis, hemoglobin likely present in urine.
    • If plasma is normal, myoglobin in urine is more likely.
    • Myoglobin is associated with muscle damage; possible causes include trauma, vigorous exercise, or overdose (e.g., alcohol).
    • Typical RBC pattern on the dipstick for intact RBCs is a dotted pattern; the amount is semi-quantified (e.g., negative, trace, small, moderate, large) or 1+, 2+, etc., depending on the lab's reporting style.
    • Interferences/limitations for blood results include:
    • Ascorbic acid (reducing agent) can cause a false negative.
    • Strong oxidizing agents can cause false positives.
    • Contamination from menstruation or hemorrhoids can cause false positives.
    • Correlate dipstick results with urine color and microscopic findings for accurate interpretation.
    • The lab workflow emphasizes mixing the urine prior to testing to prevent false negatives due to sedimentation of cells.
    • Some manufacturers provide pad overlays or dedicated pads to detect interfering substances (e.g., ascorbic acid) to alert the technologist.

  • Microalbumin (single-analyte pad for albumin):

    • Detects very small amounts of albumin in urine.
    • Albuminuria indicates potential renal concern; microalbumin is used as a sensitive marker for early renal disease.

  • Myoglobin (single-analyte pad for myoglobin):

    • Used to detect myoglobin in urine to indicate muscle breakdown.
    • Distinct from hemoglobin even when dipstick shows a positive blood result; dedicated pad helps differentiate.

  • Ketones (reagent pad) and Bilirubin (reagent pad):

    • Ketones: dipstick positive results are confirmed with the ketone tablet test.
    • Bilirubin: dipstick positive results are confirmed with the Ictotest tablet; tablets are more sensitive than dipsticks.
    • The Ictotest/Ichotest is used when dipstick reading is questionable or there’s pigment interference.

  • Reducing substances (sugar alcohols, e.g., glucose):

    • Reducing substances tablet (Clinitest) is used for confirmation; especially relevant in pediatric patients (≤2 years).

  • Notes on reporting and interpretation of analyte results:

    • Dipstick results are often reported as concentrations (e.g., mg/dL) or as qualitative categories (small/moderate/large, trace, negative, or plus signs).
    • It is not required to memorize exact reporting schemes; follow the manufacturer’s instructions and lab policy.

Practical considerations and clinical relevance

  • Why chemical analysis? It provides rapid, noninvasive screening for disease, helps monitor response to therapy, and can indicate progression.
  • The results should be interpreted in the clinical context (history, symptoms, physical exam, and microscopy).
  • Some diseases or conditions affect specific analytes (e.g., urinary albumin for kidney disease, nitrites for infection, leukocyte esterase for inflammation), so understanding pathways helps in interpretation.
  • Interfering substances and procedures must be considered to avoid misinterpretation (e.g., vitamin C, pyridium, pigment interferences).
  • The lab follows strict QC and labeling procedures to ensure accuracy and patient safety; operators must be aware of general guidelines and lab-specific protocols.

Real-world implications and best practices

  • Always correlate chemical results with urine appearance and microscopic findings for a comprehensive assessment.
  • Maintain strict sample handling to avoid degradation or contamination and ensure patient safety.
  • Be mindful of medications or dietary factors (e.g., pyridium) that can alter results; review patient history prior to interpretation.
  • Remember that certain tests (e.g., Clinitest, Ictotest) are age-specific or scenario-specific; apply guidelines accordingly.

Next topic

  • Leukocyte esterase will be discussed in the next lecture.