Sodium, being the most abundant cation in the extracellular fluid (90% of all extracellular cations) poses a vital role in the osmolality of plasma. Plasma osmolality is approximately 295 mmol/L, 270 mmol/L of which resulting from sodium and other associated anions. Having said this, sodium contributes greatly on water balance as it is the major active cation for this function.
When testing for Sodium in the blood, certain precautions are to be observed so as to preserve the specimen's integrity and obtain the actual values while eliminating external factors. These precautions include:
- Water level intake should be at a normal level (3 liters for men and 2.2 liters for females - Institute of Medicine). In normal individuals, increased water intake doesn't affect Na levels significantly. However, for diagnosing hyponatremia (decreased Na levels), excessive water intake may influence tests results because of water imbalance. Dehydration may also influence the results and exhibit false positive for hypernatremia (increased Na levels).
- Engaging in strenuous or any activity which induces sweating prior to subjecting to the test is also detrimental when it comes to testing sodium. This is due to the water loss, thus increasing the concentration of sodium which may affect the result of the test.
- Medicine or drug intake such as anabolic steroids, corticosteroids, laxatives, cough medicines, and oral contraceptives should be declared to the physician as they increase sodium levels. Also, drugs like diuretics , carbamazepine, and tricyclic antidepressants causes decreased levels of sodium. These should be monitored and be made known to avoid false postie and/or false negative results.
- Pre-existing conditions (diarrhea, profuse sweating, burns, vomiting, Addison’s disease, kidney disease, or congestive heart failure - low sodium levels; dehydration, diuretic use, Cushing’s syndrome, cystic fibrosis, neurological disorders, hypothyroidism, or renal failure - high sodium levels) that would define the Na abnormality should be noted to be able to rule out factors that are already evident upon the performance of the test.
- Human errors such improper collection and preparation of specimen, specimen contamination, and empirical treatment on the test should always be avoided by the Medical Laboratory Scientist. Also, the Universal Precaution is to be observed at all times.
Clinical Chemistry: Principles, Procedures, Correlations by Michael L. Bishop, et.al.
Henry's Clinical Diagnosis and Management by Laboratory Methods, 21st ed. by Richard A. McPherson, et.al
http://www.mayoclinic.com/health/water/NU00283
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