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Week 8: Chemical UA Sensitivity and specificity Test principles Reagent strip tests Back-up confirmatory tests Interfering substances: false positives.

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Presentation on theme: "Week 8: Chemical UA Sensitivity and specificity Test principles Reagent strip tests Back-up confirmatory tests Interfering substances: false positives."— Presentation transcript:

1 Week 8: Chemical UA Sensitivity and specificity Test principles Reagent strip tests Back-up confirmatory tests Interfering substances: false positives and negatives

2 Urine Chemistries

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4 Test Performance Sensitivity: minimum concentration Specificity: discrimination Interference factors

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7 pH Normal: (mean 6) Control blood pH Principle: Methyl red and Bromthymol blue double indicator method Report to nearest 0.5 pH unit Help identify crystals

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10 Protein Normal: negative Sensitive indicator of renal diseases Glomerulonephritis Pyelonephritis Malignant hypertension Preeclampsia Severe exercise Principle: Protein error of pH indicator Tetrabromphenyl blue buffered at pH 3.2 Watch for false positive in alkaline urine

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12 Back-Ups for Protein 3% sulfosalicylic acid (SSA) 0 to 4+ depending on precipitation Trichloroacetic (TCA) acid Heat and acetic acid

13 Microalbuminuria > 20 mg/L ( mg/day) albumin in urine Not detectable with reagent strip (6-15 mg/dL) or SSA (5 mg/dL) Predictive of nephropathy and eventual renal failure in patients with type I diabetes mellitus Detect patients with increased risk of renal and cardiovascular disease, associated with insulin resistance and endothelial dysfunction Albumin to creatinine ratio corrects for hydration level

14 Glucose Normal: negative Blood threshold for glucose 175 mg% Elevated with diabetes (hyperglycemia) Principle: Glucose oxidase and hexokinase Glu Glu Ox > Gluconic acid + H 2 O 2 H 2 O 2 + o-tolidine Perosidase > color Specific for glucose Sensitive to redox agents

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16 Back-Ups for Glucose Lily TesTape Specific to glucose Clinitest (Benedict’s) Non-specific for any reducing agents Cupper sulfate reduction Cu ++ (blue) + Glu ——> CuO (yellow-red)

17 Ketone Bodies Normal: negative Elevated with diabetic ketoacidosis, starvation Most sensitive to acetoacetate, less sensitive to acetone, not sensitive to  -hydroxy butyrate Principle: Sodium nitroprusside Legal’s test Some have glycine to increase sensitivity to acetone

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19 Back-Ups for Ketone Acetest table test Same principle as reagent strip Gerhardt’s FeCl 3 non-specific test

20 Urobilinogen Normal: up to 1 Ehrlich unit/dL (not negative) Negative in complete obstruction of common bile duct Elevated in liver disease and hemolytic anemia Principle: p-dimethylaminobenzaldehyde, azocoupling in acid to form pink azo dye

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23 Back-Up for Urobilinogen Watson-Schwartz Extraction with chloroform and butanol Not commonly done

24 Bilirubin Normal: negative Liver disease Viral hepatitis Cirrhosis Obstructive jaundice Bile stone in common bile duct Pancreatic cancer Principle: Diazonium salt reaction, azo coupling in acid to form purple azo dye

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26 Back-Ups for Bilirubin Ictotest Same principle as reagent strip Harrison’s spot test Fouchet’s reagent (TCA)

27 Blood Normal: negative Hematuria, hemoglobinuria, myoglobinuria Principle: Hb used as catalyst for o-tolidine or benzidine oxidation (pseudo-peroxidase activity) If hematuria, should see red cells Menstrual contamination Bacterial peroxidase can cause false positive

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29 Nitrite Some bacteria reduce nitrate to nitrite Diazotization with aromatic amine like p-arsanilic acid or sulfanilamide

30 Leukocyte Esterase Leukocyte (neutrophil) esterase cleave an ester which is azocoupled with aromatic amine Correlate with microscopic WBC

31 Specific Gravity Measure ionic solute As protons are released from polyelectrolytes, pH decreases that change bromthymol blue indicator

32 Ascorbic Acid Ascorbic acid reduces a dye causing color change 2,6-dichlorophenolinedophenol

33 Quality Control and Quality Assurance

34 Homework Construct a table tests: stix and back ups TestPrincipleSensSpecFalse PosFalse Neg


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