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To the Editor,We have earlier proposed that serum ACE (s-ACE) could be used as a biomarker for severity in COVID-19 due to an assumed inverse relationship between ACE and ACE2. High s-ACE could indicate lower ACE2 activity and therefore more widespread and severe SARS-CoV-2 infection, owing to virally mediated downregulation of ACE2 (1). Dysregulation of the renin-angiotensin-aldosterone system (RAAS) is found in comorbidities known as risk factors for increased morbidity and mortality, such as hypertension and cardiovascular disease (2). This dysregulation of the RAAS could be further aggravated by virally mediated downregulation of ACE2 in COVID-19, increasing the levels of pro-inflammatory and hypertensive products of ACE, for example angiotensin II (2). This further supports the hypothesis that s-ACE could serve as a prognostic biomarker in COVID-19.The aim of this prospective case series was to investigate the potential of s-ACE as a prognostic biomarker in COVID-19. S-ACE was analyzed in serum for 15 symptomatic patients admitted to Oslo University Hospital (OUH), Norway, with confirmed COVID-19 by a positive SARS-CoV-2 real-time reverse transcriptase chain reaction (RT-PCR) (Table 1). Clinical Frailty Scale (CFS) and Charlson Comorbidity Index (CCI) scores were estimated at admittance in order to stratify the patients with regard to overall fitness (CFS) and the number of comorbidities (CCI) (3). Patients using ACE inhibitors or angiotensin II receptor blockers were excluded (N = 2), as these drugs can interfere with s-ACE levels (4), leaving 13 patient for further analysis. Clinical data were registered in a COVID-19 quality registry approved by the data protection officer at OUH. S-ACE was sampled at day of admittance (N = 13) and at days 3-5 (N = 5) (Fig. 1). S-ACE was analyzed on Cobas c501 (Roche diagnostics GmbH, Mannheim, Germany) using ACE Kinetic (B€ uhlmann Laboratories AG Sch€ onenbuch, Switzerland) within 5 days from blood collection.Clinical and laboratory characteristics of the patient population are shown in Table 1. The sampled values for s-ACE both at baseline and at days

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