Clinical Chemistry 2 - Laboratory - 1 - Determination of Osmolality - Complete

Osmolality

It is a critical laboratory measurement used to assess the concentration of solutes in the blood

Osmolality

It primarily reflects the balance between body water and dissolved particles like: sodium, glucose, and urea

Osmolality

It is a critical parameter used to evaluate the body's water balance and electrolyte status.

Osmolality

It focuses on the total number of particles in a solution, which dictates the movement of water between compartments, unlike molarity or molality, which focus on the mass of a specific substance,

Osmolality

Is the concentration of solute per unit weight of solution (mmol/kg)

Osmolarity

Is the concentration of solute per unit volume of solution (mmol/L)

Osmolal gap

Is the measured osmolality minus calculated osmolarity (mmol/kg)

Osmolarity

Osmoles per liter of solution

Osmolality

Osmoles per kilogram of solvent

Osmolality

Between osmolarity and osmolality, which one is independent of temperature and pressure making it the preferred measurement in clinical laboratory settings.

Osmolality

Defined as the number of osmoles of solute per kilogram of solvent (mOsm/kg).

Osmolality

In human physiology, it represents the osmotic pressure exerted by solutes across semipermeable cell membranes

Colligative property

Osmolality is a ___, meaning it depends solely on the number of particles, not their size, charge, or chemical nature.

Freezing point, Vapor pressure, Boiling point, Osmotic pressure

Colligative properties of solutions

Decrease

If solute concentration increases, what is the effect on the Freezing point?

Decrease

If solute concentration increases, what is the effect on the Vapor pressure?

Increase

If solute concentration increases, what is the effect on the Boiling point?

Increase

If solute concentration increases, what is the effect on theOsmotic pressure?

Osmolality

It is measured by observing how particles change the physical properties of the solvent (water)

Freezing point depression

Most common clinical method of determining osmolality

Freezing point depression

Gold standard of osmolality determination

1.86°C

Each osmole of solute (1000 mOsm) per kg of water lowers the freezing point by how much?

Molal freezing point depression constant

Each osmole of solute (1000 mOsm) per kg of water lowers the freezing point by 1.86°C. What is 1.86°C known as?

Cryoscopic constant

Other name for Molal freezing point depression constant

Kf

Cryoscopic constant symbol

Vapor pressure lowering

Osmolality determination method often used for stool samples or research

Boiling point elevation and Osmotic pressure

Osmolality determination method less commonly used in routine clinical labs

The body maintains a strict equilibrium between the Extracellular Fluid (ECF) and Intracellular Fluid (ICF). Because cell membranes are highly permeable to liquid but selectively permeable to solutes, ___ acts as the primary "equalizer." It moves freely from areas of Iow osmolality to high osmolality.

Smithline-Gardner formula

Identify formula

Dolwart-Chalmers formula

Identify formula

Modified Smithline-Gardner formula

Identify formula

2(Na+) + Glucose/18 + BUN/2.8

Smithline-Gardner formula

2(Na+) + Glucose/18 + BUN/2.8 + Ethanol/4.6

Modified Smithline-Gardner formula (if with Ethanol)

Effective osmoles

Osmoles that are solutes like Sodium and Glucose that do not freely cross the cell membrane.

Ineffective osmoles

Osmoles that are solutes like Urea (BUN) that can cross membranes relatively easily.

Effective osmoles

Osmoles that "pull" water toward them (tonicity).

Ineffective osmoles

Osmoles that contribute to total osmolality but do not cause significant water shifts.

Sodium, Glucose, BUN

Primary solutes of the body

Sodium

Primary solute of the body that goes along with its associated anions (chloride and bicarbonate)

Sodium

Primary solute of the body that accounts for approximately 90% of plasma osmolality.

Glucose

Primary solute of body that is a secondary but significant contributor

BUN

Primary solute of body that is a secondary contributor

Hypothalamic-Pituitary Axis

Regulatory mechanism in the body that can detect as little as a 1% change in osmolality

True

True/False

The body maintains serum osmolality within a very narrow range (tightly regulated around 280-295 mOsm/kg) through a high-sensitivity feedback loop

Osmoreceptors

These sensors in the body detect a change as small as 1% in osmolality

Hypothalamus

Where are osmoreceptors located?

Arginine Vasopressin

This hormone is released when osmolality rises

Posterior pituitary gland

Where is AVP/ADH released?

Renal collecting ducts

It is where AVP acts on to increase water reabsorption

Thirst mechanism

Triggered by high osmolality, the thirst center in the brain prompts fluid intake

Aquaporins

High osmolality triggers the thirst mechanism, releasing ADH, which inserts ___ in the renal collecting ducts to reabsorb water

Diuresis

Low osmolality suppresses ADH, leading to excretion of dilute urine. What is the medical term for the inducing the excretion of urine?

Hypothalamus

Where is ADH produced?

Posterior pituitary gland

Where is ADH stored and released?

Hyperosmolality

Osmolality that occurs when there is a deficit of water relative to solute or an excess of solute

Hyperosmolality

Hyperosmolality or Hypoosmolality

Diabetes Insipidus

Hypoosmolality

Hyperosmolality or Hypoosmolality

Hyponatremia

Hyperosmolality

Hyperosmolality or Hypoosmolality

Hyperglycemia

Hyperosmolality

Hyperosmolality or Hypoosmolality

Toxic ingestion

Hypoosmolality

Hypoosmolality or Hypoosmolality

Syndrome of Inappropriate ADH secretion

Hyperosmolality

Hyperosmolality or Hypoosmolality

Dehydration

Hypoosmolality

Hyperosmolality or Hypoosmolality

Polydipsia

Dehydration

Cause of Hyperosmolality

Excessive loss of free water (e.g., sweating, diarrhea)

Diabetes Insipidus

Cause of Hyperosmolality

Lack of ADH or renal resistance, leading to massive dilute urine output

Hyperglycemia, Hyperosmolar Hyperglycemic State

Cause of Hyperosmolality

Seen in Diabetes Mellitus, especially in ___

Glucose

What effective osmole pulls water out of cells in Hyperglycemia?

Toxic ingestion

Cause of Hyperosmolality

Ethanol, methanol, or ethylene glycol poisoning increases the concentration of unmeasured / hidden particles.

Central or neurogenic Diabetes insipidus

Diabetes insipidus where there is lacking ADH

Nephrogenic Diabetes insipidus

Diabetes insipidus with renal resistance

Hypoosmolality

Indicates an excess of water relative to solute, leading to cellular swelling

Cerebral edema

Primary risk of Hypoosmolality

Syndrome of Inappropriate Antidiuretic Hormone secretion

Cause of Hypoosmolality

Leads to excessive water retention despite low serum osmolality.

Polydipsia

Cause of Hypoosmolality

Excessive water intake that overwhelms the kidneys' excretory capacity.

Hyponatremia

Cause of Hypoosmolality

Low sodium levels due to congestive heart failure, cirrhosis, or nephrotic syndrome (dilutional hyponatremia)

Serum

Preferred specimen for osmolality determination

Heparinized plasma

Other accepted specimen for osmolality determination

Tightly capped

How should specimens for osmolality determination handled to prevent evaporation?

2-8°C or -20°C or lower

What temperatures should specimens for osmolality determination stored at if not tested immediately?

Basic Metabolic Panel

Osmolality determination tests are often ordered alongside this panel to obtain sodium, glucose, and BUN values for gap calculation

Freezing point depression

Method of osmolality determination that measures the decrease in the freezing point.

Freezing point depression

Method of osmolality determination that is the most accurate because it detects all solutes, including volatiles like ethanol.

Vapor pressure measurement

Method of osmolality determination that measures the decrease in vapor pressure.

Vapor pressure measurement

Method of osmolality determination that is less common because it fails to detect volatile alcohols, which leads to falsely normal results in poisoning cases

Pre-analytical

Pre-analytical or Analytical error?

Hemolysis

Pre-analytical

Pre-analytical or Analytical error?

Use of volatile anticoagulants

Pre-analytical

Pre-analytical or Analytical error?

Drawing from an IV line containing dextrose or saline

Analytical

Pre-analytical or Analytical error?

Vapor pressure osmometry fails to detect volatile alcohols (ethanol, methanol), leading to a falsely low reading in toxicity cases.

Serum

Serum osmolality specimen

Sodium, Chloride, Bicarbonate, Glucose, BUN

Serum osmolality primary solutes

275-295 mOsm/kg

Serum osmolality normal range

Overall hydration, poisoning

Serum osmolality clinical focus

Thirst, ADH

Serum osmolality is controlled by?

Random or 24-hour urine sample

Urine osmolality specimen

Urea, Creatinine, Sodium, Potassium, Chloride

Urine osmolality primary solutes

50-1200 mOsm/kg

Urine osmolality normal range, highly variable

Kidney's concentrating or diluting ability

Urine osmolality clinical focus

ADH action on renal tubules

Urine osmolality is controlled by?

Serum osmolality, Urine osmolality

Comparing these two osmolality values helps pinpoint the "site of the problem" (renal vs. extra-renal)

<1.0

Urine/Serum Ratio in Diabetes insipidus because the urine is inappropriately dilute while the serum is concentrated

Free water clearance

A calculation used to determine how much water the kidney is clearing without solutes