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Specific Gravity

Specific Gravity. Refractometer Reagent Test Strip. Specific Gravity. S.G. is a measure of the density (weight) of dissolved particles in the urine Assesses the kidney’s ability to selectively reabsorb water and essential minerals Also shows patient hydration

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Specific Gravity

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  1. Specific Gravity Refractometer Reagent Test Strip

  2. Specific Gravity • S.G. is a measure of the density (weight) of dissolved particles in the urine Assesses the kidney’s ability to selectively reabsorb water and essential minerals • Also shows patient hydration • Weight of urine / Weight of water

  3. Specific Gravity • Normal range 1.005 to 1.025 • SG <1.002 or >1.040 are not physiologically possible • Measured by reagent test strip or refractometer

  4. Interference with SG tests • Glucose and Protein are high MW substances. • Their presence has nothing to do with the concentrating ability of the kidney. • If they are present in large amounts, they must be corrected for.

  5. For each gram/deciliter of protein subtract .003 from the specific gravity. • For each gram/deciliter of glucose subtract .004 from the specific gravity. • Example: • 2 gm/dl Glucose - .008 • 3 gm/dl Protein - .009 • Specific Gravity = 1.052 • Correction - .017 • Corrected SG = 1.035

  6. High S.G. dehydration high levels of glucose or protein x-ray contrast media or dye diabetes mellitus Low S.G. dilute urine renal failure tubular necrosis diabetes insipidus decreased ADH function Specific Gravity

  7. Urometer • Old Technology • Uses too much urine • Affected by large molecules in the urine • We don’t do it anymore! • Needs correction for Temp and Glucose & Protein

  8. Refractometer QC • Distilled Water 1.000 • 5% NaCl 1.022 +/- .001 • 9% Sucrose 1.034 +/- .001

  9. SG – Reagent Test Strip • Indirect colorimetric estimation of the specific gravity • Detects only the ionic solutes • No need to correct SG for glucose, etc. • Reagent Pad • Polyelectrolyte: pH indicator • Maintained at an alkaline pH • Immerse pad in urine the pKa of the polyelectrolyte will decrease with ionic concentration

  10. SG – Reagent Test Strip • Bromthymol Blue: indicator • As pH decreases color change • From dark blue-green (SG - 1.000) to yellow-green (SG - 1.030) • More ions – more protons released from polyelectrolyte – decrease pad pH – change in indicator.

  11. S.G. Test Strips Interference • False Positive High Concentration of Protein • False NegativesHighly Alkaline urine (>6.5)pH >6.5 add .005 to reading

  12. Refractometer Correct for Glucose and Protein Test Strip No interference from large organic molecules, glucose, urea, radiographic contrast media, plasma expanders S.G. Refractometer vs Reagent Strip

  13. Why does a urine with a low specific gravity produce an alkaline reaction with bromthymol blue in the S.G. test? • Hydrogen ions are released from the polyelectrolyte in proportion to the specimen concentration

  14. How do specific gravity readings differ between reagent strips and refractometers? • Reagent strips are not affected by nonionizing high MW substances.

  15. Explain the need to add 0.005 to the specific gravity readings in urines with a pH of 6.5 or higher • The alkaline pH of the urine requires additional hydrogen ions to be released from the polyelectrolytes to produce a color change: therefore the true concentration is not represented.

  16. Osmolality • Is the concentration of a solution expressed in osmoles of solute particles per kilogram of solvent. • One osmole is the amount of a substance that dissociates to produce one mole (6.023 x 1023) of particles. NaCl(One mole of salt) Na+ Cl- (Two osmoles) Glucose (One mole) Glucose (One osmole)

  17. Osmolality • Performed by Freezing Point Depression. • The freezing point is inversely proportional to the concentration of the urine. • Dependant upon number of particles not size or ionic charge.

  18. Reagent Test Strip Reactions Based on three principles: 1.Colorimetric: substance in urine plus key color = visible reaction 2.Enzymatic: enzymes breakdown specific compounds = visible reaction 3.Catalytic: substance in urine causes reaction to proceed = visible reaction

  19. Reagent Test Strips Two different types of information result: 1.Qualitative: positive or negative 2.Quantitative: how much (mg/dl, 4 plus, etc.)

  20. Reagent Test Strips • Run a positive and negative control daily • Record results • Repeat if controls are out of range • Date bottles when received and when in use • QC new Lot #’s of reagents and strips

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