Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1223 page)

   Limitations and Interferences

   Plasma Na levels depend greatly upon the intake and excretion of water and to a somewhat lesser degree the renal regulation of Na.

   Determinations of blood sodium and potassium levels are not useful in diagnosis or in estimating net ion losses but are performed to monitor changes in sodium and potassium during therapy.

   Hyperglycemia—serum sodium decreases 1.7 mEq/L for every increase of serum glucose of 100 mg/dL.

   Hyperlipidemia and hyperproteinemia, which cause spurious results only with flame photometric but not with specific ion electrode techniques for measuring sodium.

   Pseudohyponatremia caused by “water exclusion effect” observed on indirect ISE (ion-selective electrode) measurements due to the dilution of samples and transfusion of blood products and due to infusion of IV immunoglobulins.

SODIUM, URINE

   Definition

   Urinary sodium determinations are usually performed to detect or confirm the presence of conditions that affect body fluids (e.g., dehydration, vomiting, and diarrhea) or disorders of the kidneys or adrenal glands.

   
Normal range:

   Twenty-four–hour urine:

   Male:

•   Less than 10 years: 41–115 mmol/day

•   10–14 years: 63–177 mmol/day

•   Greater than 14 years: 40–120 mmol/day

   Female:

•   Less than 10 years: 20–69 mmol/day

•   10–14 years: 48–168 mmol/day