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