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Comment on “Association of Serum Osmolarity With Contrast-Induced Nephropathy in Patients With ST-Segment Elevation Myocardial Infarction”

We read with interest the article by Yildiz et al entitled “Association of serum osmolarity with contrast-induced nephropathy in patients with ST-segment elevation myocardial infarction.” We agree that increased osmolarity is a strong risk factor for contrast-induced nephropathy (CIN). Recently, Kim et al determined a relationship between CIN and osmolar gap. The findings of these 2 studies support each other. However, we have some concerns about the methodology used in the Yildiz et al’s study. Serum osmolality is detected primarily by sodium (Na), its counter ions, and uncharged species such as glucose and blood urea nitrogen (BUN). In the study of Yildiz et al, plasma osmolarity was calculated with the formula 2Na + glucose + BUN (all in mmol/L) or 2Na (mmol/L) + glucose (mg/dL)/18 + BUN (mg/dL)/2.8. The osmolar gap is obtained by subtracting the osmolarity value calculated from the osmolality measured by an osmometer. Increased ethanol, methanol, and some molecules in plasma may cause an elevated osmolar gap. Osmolarity calculated by the formula often does not provide a directly measured osmolarity value. It is doubtful whether their patients consumed alcohol within 24 hours. Also, hyperlipidemia, hyperglycemia, and hypoproteinemia may result in low Na measurement, leading to calculate false low osmolarity. It seems that no fasting blood samples were taken from the patients. Triglyceride levels may have affected the osmolarity results. Psychogenic polydipsia, syndrome of inappropriate antidiuretic hormone secretion, hypothyroidism, or states of volume overload (congestive heart failure, cirrhosis, nephrotic syndrome) are causes of hypotonic hyponatremia. In this study, lipid profile, thyroid-stimulating hormone, urine protein, and urine osmolality were not evaluated. Blood gas testing helps to rule out other causes that increase osmolarity, such as acid– base disorders, anion gap acidosis, ethanol toxicity, lactate level, or diabetic ketoacidosis. One-fourth of their patients with CIN had diabetes. Recent studies have reported that metformin is not associated with CIN. Recently, many studies reported that metformin is not causing lactic acidosis. However, metformin can insignificantly increase blood lactate levels. Increased lactate levels (cutoff value 1.5 mmol/L) may raise the osmolar gap. It is not known whether Yildiz et al measured serum lactate levels of patients. The use of sodium–glucose cotransporter 2 inhibitors has increased in heart failure as well as diabetes. Sodium–glucose cotransporter 2 inhibitors may cause euglycemic diabetic ketoacidosis; as a result, the osmolar gap may increase. It is not known whether patients had euglycemic ketoacidosis in the Yildiz et al’s study. The number of hypertensive patients in the CIN group is high. It has not been reported whether these patients used diuretics. Diuretics can increase osmolarity by causing dehydration. Some CIN group patients had low blood pressure and high pulse rates. These findings support dehydration. In this study, skin turgor and mucous membrane feature of the patients were not specified. Besides, hematocrit values were not given. A previous study reported that patients receiving the 3 g salt showed a 6 mOsm/L change in osmolality, with a 2.5 mmol/L change in plasma sodium at 2 hours. Have their patients received any salty food or solution a short time before admission? Even a small amount of salt intake can change osmolarity. Suckling et al reported that 6 g salt intake in a single meal of normotensive individuals increased plasma osmolarity by 3.80 + 0.58 mOsmol/L at 60 minutes. Glucose levels are reduced in old blood samples due to glycolysis. Have their patients’ blood samples been studied immediately in the laboratory? Finally, we speculate that it is not appropriate to study osmolarity alone in patients with CIN. Calculated osmolarity is not always the correct value. Osmolar gap and anion gap calculation with proper tests are required. Detailed prospective studies are needed to clarify the relationship between osmolarity and CIN.