CLS 1113 Introduction to Clinical Laboratory Practices

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CLS 1113Introduction to Clinical Laboratory
Practices

• Specific Gravity
• The Reagent Test Strip

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Specific Gravity (Chapter 56)

• S.G. is a measure of the density (weight) of
  dissolved particles in the urine
• Assesses the kidneys ability to reabsorb water
  and essential minerals
• Also shows patient hydration
• Weight of urine / Weight of water

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Specific Gravity

• Normal range 1.005 to 1.025
• SG lt1.002 or gt1.040 are not physiologically
  possible
• Measured by reagent test strip or refractometer

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Interference with SG tests

• Glucose and Protein are actively REABSORBED in
  the nephron.
• 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.

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• 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

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Specific Gravity

• Low S.G.
• dilute urine
• renal failure
• tubular necrosis
• diabetes insipidus
• decreased ADH function

• High S.G.
• dehydration
• high levels of glucose or protein
• x-ray contrast media or dye
• diabetes mellitus

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Urometer

• Old Technology
• Uses too much urine
• Affected by large molecules in the urine
• We dont do it anymore!

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Refractometer

• Measures the velocity of light in air compared to
  the velocity of light in a solution. This is the
  refractive index.
• The refractive index is converted to Specific
  Gravity on the refractometer.

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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

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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.

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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)
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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.

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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

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Reagent Test Strips

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

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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