Plasma LDH: A specific biomarker for lung affectation in COVID-19?

OBJECTIVES: We aimed to determine whether the plasma profile of lactate dehydrogenase (LDH) isoenzymes is altered in patients with COVID-19, and whether this is attributable to a specific release of LDH-3, the main LDH isoenzyme expressed in lungs. DESIGN: We collected fresh plasma aliquots from 17 patients (LDH range, 281–822 U/L) and seven controls (LDH ​< ​230 U/L). In-gel relative activity of the different LDH isoenzymes was determined by electrophoresis and densitometric analysis. RESULTS: Despite the expected higher total LDH activity levels in patients (p ​< ​0.001), the in-gel relative activities of LDH isoenzymes did not differ between patients and controls (all p ​> ​0.05). We found no correlation between total plasma LDH activity and the in-gel relative activities of the different LDH isoenzymes, including LDH-3. Likewise, there was no correlation between LDH-3 and various routine haematological and serum parameters that have been previously reported to be altered in COVID-19 (such as lymphocyte count, albumin, alanine and aspartate aminotransferase, creatinine, C-reactive protein, or ferritin). CONCLUSIONS: Our findings suggest that elevation of plasma LDH activity in patients with COVID-19 is not associated to a specific release of LDH-3 into the bloodstream, and do not support the use of LDH as a specific biomarker for lung affectation in patients with COVID-19.