APP NOTES

THE VALUE OF OSMOLALITY TESTING

Neville R. Dossabhoy, MD, FACP, FASN Consultant Nephrologist

ABSTRACT

Measured osmolality is a great initial screening tool when evaluating patients with metabolic acidosis, ingestions with various alcohols and alcohol-derivatives, common intoxications, and disorders of plasma sodium. It helps narrow down the differential diagnoses for these conditions in a cost- and time-effective manner. Hospitals and health systems can include measured osmolality in clinical pathways for these conditions to help reduce cost of care and improve quality of care.

DESCRIPTION One such test is measured osmolality. dozen (see Figure 1). This leads to This can help narrow the differential significant cost savings and a quicker Some common conditions patients diagnoses for the following conditions: and proper diagnosis, and eliminates present with in emergency rooms, the patient spending days in an ICU hospital wards, and ICU’s are 1. Ingestions with various alcohols bed waiting for results of several ingestions with various alcohols and alcohol-derivatives (e.g., dozen tests. From a cost-analysis point and alcohol-derivatives, common methanol, ethanol, ethylene of view, just one or two patient stays in intoxicants, metabolic acidosis, and glycol, propylene glycol, ICU can cost as much as the cost of the disorders like hypo- and isopropyl alcohol) instrument that measures osmolality hypernatremia. The differential (osmometer) – approximately USD diagnosis for each of these conditions 2. Common intoxicants like toluene, 40-50K. – namely the list of possible causative salicylates, ethylene glycol, agents – can be long, and involve propofol, isopropyl alcohol; as several dozen possibilities. also iatrogenic overdosing with mannitol, sorbitol, and Consider metabolic acidosis, for glycine (commonly used during instance; the list of possible causes surgical procedures like (called “differential diagnosis “) can neurosurgery and TURP) be over one hundred. It is highly expensive to test the lab analyte for 3. Metabolic acidosis – especially of each possible cause. Additionally, the high variety such a process of elimination would (HAGMA for short) be time-consuming, cost-ineffective, and probably clinically unhelpful. What 4. Electrolyte disorders like is needed in such situations are one hypo- and hypernatremia or two tests that can help narrow the differential diagnosis to a manageable The measurement of plasma osmolality list – which can then be tested in a helps to calculate the “Osmolal Gap”. more cost-effective and time-efficient Combining the use of the Anion Gap manner. and the Osmolal Gap helps narrow the list of differential diagnosis for HAGMA from a hundred plus, to less than a

1 counting the possible costs arising to have the four broad conditions Measurement of the plasma from litigation for malpractice by enumerated above. Quick and proper osmolality is also crucial to making misdiagnosis and improper treatment. diagnosis helps decide whether the the correct diagnosis in patients with Similar logic applies for patients with patient needs to be admitted to the (see Figure 2). The Hypernatremia. hospital; if so, to which type of bed possibilities are completely different - (ICU versus ward); and thereafter help and so is the proper treatment – The use of osmolality and Osmolal decide what appropriate therapy to depending on whether osmolality is Gap as a screening test for certain provide; etc. The actual construct of low, normal, or high. It would be very defined conditions – like the ones the pathway would likely depend on easy to make the wrong diagnosis enumerated above – can lead to the the specific site at which the testing is if the osmolality were not measured construction of clinical pathways. being done (ER, ICU, ward, clinic, etc). or not available, and thereby end These pathways would be akin to the up giving the wrong treatment to workup of patients suspected of having It is obvious that such pathways the patient. Not only would that be pulmonary embolism, for instance, will yield benefits in terms of bed potentially harmful to the patient; but whereby the D-dimer and the V/Q utilization; and thereby can help also very expensive, given that some scan help both in making the proper reduce the cost of care and improve of the medications now available diagnosis, as well as guide the proper the quality of care. The cost savings to treat SIADH (one of the causes therapy. Another example would be would accrue at the level of the of hyponatremia) cost literally USD the clinical pathway for DVT, using the patient, the hospital, the hospital 500-1000 per day! Just one or two D-dimer and Duplex ultrasound. The system, the insurer, and eventually the misdiagnosed patients can cost osmolality could be used similarly, health care system. the hospital system as much as the incorporated into a clinical pathway price of the osmometer! This is not or algorithm, in patients suspected

HYPONATREMIA HIGH OSMOLAL GAP (OG)

OG = Measured osm - Calculated osm Iso-osmolar Hypo-osmolar Hyper-osmolar Normal OG = 10; if OG > 25, suspect presence of ingestion S osm 280-295 S osm < 280 S osm > 295 Uso to narrow the differential diagnosis

HIGH OSMOLAL GAP Euvolemia Hypovolemia Hypervolemia With HAGMA Without HAGMA

Methanol Isopropyl alcohol SIADH DIURETICS FAILURE Ethylene glycol Mannitol ADRENAL INSUFF OSMOTIC DIURESIS CARDIAC FAILURE Ethanol Sorbitol HYPOTHYROIDISM VOMITING/DIARRHEA CIRRHOSIS Lactic acidosis Glycine NEPHROTIC SYNDROME Ketoacidosis Maltose Figure 2: Differential Diagnosis of HYPONATREMIA **glycine, sorbitol, mannitol used for transurethral resection of prostate or bladder tumor

Figure 1: Osmolal Gap can help narrow the Differential Diagnosis of HAGMA

To learn more, see the other articles in this series: The Value of Osmoloity Testing in Nephrology The Value of Osmoloity Testing in Emergency Medicine by Dr. Namrata Sharma Goel by David Schindler, M.D.

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