Q&A column

Editor: Frederick L. Kiechle, MD, PhD

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Q. Is it necessary for a lab to report a corrected sodium level when the glucose level is really high? Studies show pseudohyponatremia can occur due to hyperglycemia. How common is this, and how do we decide which correction factor to use? Is it possible that this is easily overlooked by providers due to comorbidities in patients? Some references say there is a need to correct glucose for each 100 mg/dL increase above 400 mg/dL glucose.

A.August 2021—Sodium (and its accompanying anions), glucose, and urea are the three major contributors to plasma osmolality. But only sodium and glucose contribute to effective osmolality (also called tonicity). Because sodium and glucose cannot cross cell membranes without active transport, both solutes influence the movement of water between the intracellular and extracellular compartments in ways that urea, which can passively move across cell membranes, does not.

The phenomenon of pseudohyponatremia, usually due to increased lipid or very high protein concentrations, is essentially a laboratory artifact caused by measuring a diluted sample and not accounting for the reduction in plasma water. Hyponatremia caused by hyperglycemia, on the other hand, is a real reduction in the sodium concentration. Elevated glucose causes a shift of intracellular water into the extracellular space, diluting sodium. Glycosuria also results in a loss of sodium and water. During the process of treating hyperglycemia, physicians must replace the patients’ sodium and fluid that has been lost. How quickly, and with what fluid, requires monitoring a patient’s plasma osmolality, but sodium levels are also often used. Consequently, there is sometimes interest in correcting the sodium level for the degree of hyperglycemia in order to guide the replacement of sodium and water as the glucose level falls.

Hyponatremia due to hyperglycemia appears to be relatively common and relatively well-recognized by endocrinologists and critical care physicians. Most caregivers add approximately 2 mmoL/L to the patient’s sodium level for each 100 mg/dL increase in glucose above 100 mg/dL. However, an accurate calculation needs to take many factors into consideration, including the level of glucose (the slope of the hyponatremia changes as the level of glucose rises above 400 mg/dL) and, if present, the degree of a patient’s obesity, which may alter distribution of body water. In addition, fluid and electrolyte balance can be complicated in critically ill patients, especially patients with diabetes mellitus who have ketoacidosis or hyperosmolar coma.

Several calculations have been proposed in the literature and there is no consensus as to which one is correct, perhaps because no one calculation can apply to all patients due to such factors as those previously mentioned. Therefore, it is probably best for laboratories to allow patient caregivers to decide which correction calculation to use versus report a corrected sodium level. Primum non nocere!

Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction factor for hyperglycemia. Am J Med. 1999;106(4):399–403.

Wolf MB. Hyperglycemia-induced hyponatremia: reevaluation of the Na+ correction factor. J Crit Care. 2017;42:54–58.

James D. Faix, MD
Medical Director
Clinical Chemistry and Immunology
Montefiore Medical Center
Bronx, NY
Member, CAP Clinical Chemistry Committee

Q. Payers are limiting reimbursement for PCR respiratory panels to a small subset of tested pathogens and only with certain indications. Many panels available from manufacturers test for more pathogens than can be reimbursed. What is the best approach to deal with this issue? Should large respiratory panels no longer be offered? If a large panel is performed, should only a limited number of pathogen results be reported, even though the entire panel was performed? Should the entire panel be reported but only a limited number of pathogens billed for?

A.If the PCR respiratory panel is designed to be run all at once, then it is not easy to run only a few tests at a time. Laboratories or ordering physicians should make a medically informed decision to order only what they feel is necessary. This does not mean the payers will agree, but the laboratory or ordering physician needs to make the decision based on science, not reimbursement.

Telcor recommends that its customers bill for the entire panel and expect the denial. This may require separate service lines for covered and noncovered tests. Best practice is to bill exactly for what was tested and resulted. Many payers will reimburse for the covered tests and deny the noncovered tests. It is important to take the adjustment once the remit is denied so time is not spent on dollars that will not be recouped. If a payer denies or rejects the entire claim, limit what goes on the resubmitted claim so the covered charges are not denied along with the noncovered charges.

If the payer rules do not match what is medically necessary for a patient, then follow the appeals process to justify the additional tests and try to get payers to change. This is not an easy process, but it is sometimes necessary.

Kwami Edwards
Chief Customer Officer, Telcor
Lincoln, Neb.