Sodium is an important electrolyte in the body that helps regulate fluid balance, nerve transmission, and muscle function. Testing sodium levels is an important part of routine blood work to monitor electrolyte status. When collecting blood samples for sodium testing, it is essential to use the proper tube to avoid erroneous results.
Sodium Blood Test Overview
A sodium blood test measures the amount of sodium dissolved in blood serum or plasma. Sodium levels are normally expressed in millimoles per liter (mmol/L). Normal sodium range is typically 135-145 mmol/L.
| Parameter | Normal Range |
|---|---|
| Sodium | 135-145 mmol/L |
Sodium balance in the body is primarily regulated by the kidneys. Too much sodium can lead to fluid retention and swelling, while too little can cause cellular dehydration. Conditions like dehydration, adrenal insufficiency, heart failure, cirrhosis, and kidney disease can impact sodium levels. That’s why checking sodium is important for evaluating patients’ fluid and electrolyte status.
Sample Collection Tube Types
There are a few main types of blood collection tubes used for laboratory testing:
| Tube Type | Tube Color | Additives | Uses |
|---|---|---|---|
| Serum separator tube | Red/gray | Clot activator and gel separator | Serum testing |
| Plasma separator tube | Green | Lithium heparin additive | Plasma testing |
| Sodium citrate tube | Light blue | Sodium citrate additive | Coagulation testing |
| EDTA tube | Lavender | EDTA additive | Whole blood testing |
The tube additives and separation gels help preserve the sample for accurate test results. Using the correct tube type is imperative for sodium testing.
Serum Separator Tubes
For serum sodium measurement, serum separator tubes with clot activator and gel are used. These tubes have a tiger top or speckled red/gray rubber stopper.
The clot activator promotes fast clotting so the blood can separate into serum and cells. The gel acts as a barrier between the serum and cells after centrifugation. This provides clear, uncontaminated serum for analysis.
Serum separator tubes are ideal for sodium testing because sodium is measured in the serum portion of blood. Sodium results would be falsely lowered if measured in whole blood rather than just serum. The gel barrier ensures only serum is sampled, not cellular components.
Sodium Tube Additives
Certain additives like heparin or EDTA cannot be used to test serum sodium.
Heparin tubes contain the anticoagulant heparin which binds to sodium. This would interfere with accurate sodium measurement.
EDTA chelates calcium but also partially binds sodium. It would cause a small but significant decrease in measurable sodium levels.
Therefore plain serum tubes with inert clot activator, or tubes with thrombin or silica clot activator are suitable for sodium testing. Tubes should be free of sodium-binding additives.
Sample Stability
Once collected, serum sodium samples are stable for up to 7 days refrigerated at 2-8°C. If storage longer than 7 days is needed, serum samples can be frozen at -20°C for up to 3 months. Repeated freeze-thaw cycles should be avoided.
Delivering serum specimens promptly to the laboratory helps ensure integrity of sodium results. If delays are expected, storing chilled or frozen can extend stability.
Sodium Reference Range
The reference range for serum sodium is 135-145 mmol/L. Normal sodium levels ensure proper fluid balance and cellular function.
| Condition | Sodium Level |
|---|---|
| Normal | 135-145 mmol/L |
| Hyponatremia | Below 135 mmol/L |
| Hypernatremia | Above 145 mmol/L |
Mild sodium imbalances are often asymptomatic. More severe hyponatremia or hypernatremia can cause neurological symptoms. Accurately assessing sodium status from blood samples is key for diagnosis and treatment.
Causes of Abnormal Sodium Levels
Many medical conditions can alter serum sodium levels.
Hyponatremia Causes
Hyponatremia occurs when serum sodium drops below 135 mmol/L. Causes include:
- Dehydration from diarrhea, vomiting
- Overhydration from IV fluids, kidney failure, cirrhosis, heart failure
- Adrenal insufficiency
- Syndrome of inappropriate antidiuretic hormone (SIADH)
- Hypothyroidism
- Kidney disease
- Diuretic medications
Hypernatremia Causes
Hypernatremia is elevated sodium above 145 mmol/L. Causes include:
- Dehydration from insufficient fluid intake
- Sodium bicarbonate therapy
- Hyperaldosteronism
- Diabetes insipidus
- Excessive sodium intake
- Osmotic diuresis
- Burns
Identifying the underlying cause of hypo- or hypernatremia is needed along with correcting the sodium abnormality.
Clinical Significance
Sodium testing provides important clinical information:
- Fluid and electrolyte balance – Sodium is the major cation in extracellular fluid and helps regulate distribution of water between cells and blood.
- Kidney function – Kidneys control sodium reabsorption and excretion to maintain normal serum sodium.
- Endocrine function – Hormones like ADH, aldosterone, and thyroid hormone influence sodium levels.
- Acid-base balance – Sodium interacts with bicarbonate and chloride to impact blood pH.
- Neuromuscular function – Normal sodium is vital for generating nerve impulses and muscle contraction.
- Hydration status – Abnormal sodium may indicate water excess or deficit.
Sodium testing aids diagnosis of electrolyte abnormalities, fluid disorders, kidney disease, endocrine dysfunction, and more. It provides key information about patients’ acid-base balance, volume status, and neuromuscular function.
Treatment of Hyponatremia and Hypernatremia
Treatment depends on the underlying cause and severity of sodium disturbance:
Hyponatremia Treatment
- Restricting fluids to raise sodium concentration
- Medications to promote aquaresis like loop diuretics
- Sodium tablets or IV saline to replace sodium
- Addressing underlying causes like heart failure or hypothyroidism
Hypernatremia Treatment
- Water replacement with oral liquids or IV fluids
- Restricting sodium intake
- Managing causes like uncontrolled diabetes or primary adrenal disorders
Too rapid correction of chronic hyponatremia or hypernatremia can cause brain swelling or shrinkage. Thus sodium levels should be normalized carefully under medical supervision.
Conclusion
Sodium testing requires using plain serum separator tubes without sodium-binding additives. This allows accurate measurement of sodium concentration in serum. Reference range for sodium is 135-145 mmol/L. Mild sodium imbalances may have no symptoms initially but can progress if left untreated. Diagnosing and managing hypo- or hypernatremia relies on trends in sodium levels over time, clinical presentation, and integration with other lab test findings. Sodium testing provides vital insights into patients’ fluid and electrolyte status.
References
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