1. Refractometer Reagent Test Strip
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
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
3 gm/dl Protein
Specific Gravity = 1.052
Correction
Corrected SG = 1.035

6. Specific Gravity High S.G. dilute urine renal failure
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

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
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.

9. 4/19/2017
Temp is not critical

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

12. 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

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

15. S.G. Test Strips Interference
False Positive High Concentration of Protein
False Negatives Highly Alkaline urine (>6.5) pH >6.5 add .005 to reading

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

17. 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

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

19. 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.

20. 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)

21. 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.

22. 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

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

24. 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