L33: Osmolality & Urinary Specific Gravity

Osmolality

total solute concentration /kg of water

Osmolarity

total solute concentration /L of water

Urine specific gravity

measures urine solute concentration using refractive index

Refractometer

measures serum & plasma protein, or urine solute concentrations by refractive index

Osmometer

measures solute concentration in plasma based on depression of freezing point

Feline dehydration USG should be

>1.040

Canine dehydration USG should be

>1.030

Large animals dehydration USG should be

>1.025

What does the refractometer measure in urine samples of healthy patients?

urine specific gravity due to refractive index of total solutes

What analytical teat assesses urine solute concentration by refractometry?

urine specific gravity

How can you estimate osmolality from USGref?

use last two digits x 30

USGref overstimates the solute concentration if there is

high protein (> 4+) or glucose (> 3+)

What substances when increased can overestimate solute concentration by increasing USGref?

proteins & glucose

Glomerular filtration (passive)

substances filter from plasma to tubules (excluding proteins/Albumin)

What tubular segment reabsorbs water under the influence of ADH?

collecting duct

Osmolality in each segment of the nephron

Proximal collecting tubule

removes volume, no change in urine concentration

- 75% water reabsorbed

- 75% solute reabsorbed

- osmolality of proximal tubular fluid is same as plasma (300 mmol/kg)

Descending loop of Henle

removes water, increases solute concentration

- urine becomes more concentrated (higher osmolality 1500 mmol/kg)

Ascending loop of Henle

removes solute, dilutes solute concentration

- removes Na, K, Cl, fCa, Mg

- water stays in tubules, urine becomes more dilute

Collecting duct

ADH stimulates removal of water - increased concentration & osmolality

- if does not remove water - stays dilute

To produce concentrated urine

1. ADH must be present (from posterior pituitary)

2. tubules must respond to ADH

3. concentration gradient must be present

What portion of the nephron reabsorbs water and electrolytes resulting in osmolality same as plasma?

proximal convoluted tubule

For kidneys to produce a urine with a specific gravity of 1.001-1.003 or osmolality of ~100 mmol/kg, what nephron segment must be functional?

ascending loop of Henle (diluting limb of the nephron)

For kidneys to produce a urine with a specific gravity of 1.030 (osmolality ~1500 mmol/kg), which portion of the nephron must be functional?

all segments must be functional

Hypersthenuria

very concentrated, high USG

dogs: >1.050

cats: >1.060

Eusthenuria

concentrated

dogs: 1.015-1.045

cats: 1.035-1.060

Isosthenuria

same osmolality as plasma

1.007-1.013

Hyposthenuria

low concentration, low USG

< 1.007

In dehydrated dogs, hypersthenuria should be above

1.030

In dehydrated cats, hypersthenuria should be above

1.040

Proteinuria is caused by

damaged filtration barrier (glomerulus)

Glucosuria is caused by

diabetes mellitus

High USGref =

low urine volume

Low USGref =

high urine volume

> 1.030 for dogs and >1.040 for cats is evidence of

adequate concentrating ability in dehydrated states

In what context do you interpret USGref?

hydration status & treatment, azotemia, polyuria, oliguira or anuria

UA results: 1.035, other findings negative. What is the conclusion?

hydration status: normal

blood volume: OK

plasma osmolality: OK

ADH release: ↑

tubule response to ADH: OK

concentration gradient: OK

water conservation: OK

conclusion: conserving water

UA results: 1.060, dehydrated. What is the conclusion?

hydration status: dehydration

blood volume: ↓↓

plasma osmolality: OK

ADH release: ↑↑

tubule response to ADH: ↑↑

concentration gradient: OK

water conservation: ↑↑

conclusion: conserving water in a dehydrated state

UA results: 1.003, other findings negative. What is the conclusion?

hydration status: overhydration

blood volume: ↑

plasma osmolality: OK

ADH release: No

tubule response to ADH: N/A

concentration gradient: OK

water conservation: ↓↓

conclusion: hyposthenuric- ascending LOH functioning, do not need ADH

UA results: 1.010, well hydrated healthy animal. IV fluids or drinks H20. What is the conclusion?

hydration status: well hydrated

blood volume: ↑

plasma osmolality: OK

ADH release: little

tubule response to ADH: N/A

concentration gradient: OK

water conservation: ↓

conclusion: isosthenuric value due to well hydration state

UA results: 1.010, dehydrated animal. What is the conclusion?

hydration status: dehydrated

blood volume: ↓↓

plasma osmolality: OK

ADH release: ↑↑

tubule response to ADH: poor (due to nephron damage)

concentration gradient: poor

water conservation: ↓↓

conclusion: isosthenuric value, unable to concentrate when dehydrated most likely due to nephron damage

UA results: 1.025 (eusthenuria), 4+ glucose = 1.015. What is the conclusion?

hydration status: dehydrated

blood volume: ↓↓

plasma osmolality: ↑↑ (glucose)

ADH release: ↑↑

tubule response to ADH: ↑

concentration gradient: ↓↓

water conservation: not enough for dehydration

conclusion: osmotic diuresis / medullary washout

UA results: 1.020, dehydration, hyponatremia, hypochloremia. What is the conclusion?

hydration status: dehydrated

blood volume: ↓↓

plasma osmolality: ↓↓ (NaCl)

ADH release: ↑↑

tubule response to ADH: ↑

concentration gradient: ↓ (NaCl)

water conservation: ↓↓

conclusion: hypoadrenocorticism (Addison's), NaCl necessary for medullar concentration gradient

UA results: 1.002, other findings negative, hyposthenuric value. What is the conclusion?

hydration status: dehydrated

blood volume: ↓↓

plasma osmolality: OK

ADH release: ↑↑

tubule response to ADH: No

concentration gradient: OK

water conservation: ↓

conclusion: CT not responding to ADH (renal diabetes insipidus), hypercalcemia/pyometra/liver failure

UA results: 1.002, dehydrated, hyposthenuric value. What is the conclusion?

hydration status: dehydrated

blood volume: ↓↓

plasma osmolality: OK

ADH release: No

tubule response to ADH: OK

concentration gradient: OK

water conservation: ↓

conclusion: lack of ADH from brain- central diabetes insipidus

A cat presents with a USGref of 1.065. What condition is most likely causing the USG result?

dehydration: hypersthenuria > 1.060 (very concentrated urine)

A cat presents with mild dehydration. The cat was treated at a veterinary clinic with IV fluids. Following the treatment, USG was 1.002. What condition is most likely causing the USG result?

overhydration: from fluid therapy, dilute urine (1.002)

A cat presents with dehydration and glucosuria of 4+ and USG of 1.025. What condition is most likely causing the USG?

osmotic or solute diuresis: from diabetes (water follows solutes), isosthenuria

A cat presents with severe dehydration and anorexia for three months with oliguria. USG is 1.011. What condition is most likely causing the USG result?

damage to nephrons: kidney disease

An intact-female-dog presents for fever, inflammatory leukogram, and high urine volume with USG of 1.003. What condition is most likely causing the USG?

collecting tubule not responding to ADH: endotoxins from pyometria inhibit tubular response to ADH