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 Weight of urine / Weight of water
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
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.
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
Specific Gravity
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
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
Refractometer QC
Distilled Water 1.000 5% NaCl 1.022 +/- .001 9% Sucrose 1.034 +/- .001
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
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.
S.G. Test Strips Interference
False Positive High Concentration of Protein
False NegativesHighly Alkaline urine (>6.5)pH >6.5 add .005 to reading
S.G. Refractometer vs Reagent Strip
Refractometer Correct for Glucose
and Protein
Test Strip No interference from
large organic molecules, glucose, urea, radiographic contrast media, plasma expanders
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
How do specific gravity readings differ between reagent strips and refractometers?
Reagent strips are not affected by nonionizing high MW substances.
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.
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)
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.
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
Reagent Test Strips
Two different types of information result:
1. Qualitative: positive or negative
2. Quantitative: how much (mg/dl, 4 plus, etc.)
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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