Earlier this year ferritin was highlighted by the IFCC as one of the abnormal diagnostic markers in COVID-19 patients. By now, several studies from all over the world have proven that elevated serum ferritin levels are associated with increased mortality rate in COVID-19 patients. [1, 2, 3]
A retrospective study from China showed that a high level of serum ferritin is an independent risk factor for the severity of COVID-19. 
Measurement of serum ferritin is an early and easily available screening tool for the assessment of disease severity in hospitalized COVID-19 patients. Therefore, serum ferritin measurement could play a crucial role in identifying patients with a higher risk of an unfavorable disease outcome.
One study found that even COVID-19 patients that experienced no symptoms showed increased serum ferritin values. The ferritin levels remained stable or decreased very slowly during the whole course of disease in the five asymptomatic patients. 
Ferritin acts as a pathogenic mediator by enhancing the inflammatory process in other inflammatory disease such as adult-onset Still´s Disease, Macrophage activation syndrome and sepsis. In addition, hyperferritinemia was associated with a severe disease progression in other viral diseases such as dengue fever. It is possible that ferritin has a pathogenic role through stimulation of the inflammatory pathways in COVID-19, too. Therefore, a therapeutic approach targeting ferritin is discussed as an option for treatment in COVID-19. Techniques such as therapeutic plasma exchange could be beneficial as it can decrease ferritin and cytokine levels. [6, 7]
Finally, researchers found that more than serum ferritin levels alone, the ratio of glycosylated fraction of ferritin (GF) could be of great interest in the diagnosis of COVID-19. In a study of 58 hospitalized COVID-19 patients the GF-rate was a highly sensitive marker and was even better at identifying severe cases than ferritin determination alone. The researchers used a combination of lymphocyte count (<540/mm3), procalcitonin (≥0.5 μg/L), and GF rate (≤40%) to establish a severity score (one point each). In their study, a score ≥ 2 provided a sensitivity of 71% and a specificity of 95% for disease severity. 
For information on DiaSys Ferritin and Procalcitonin, please refer to:
1. Tural O, et al. Could ferritin level be an indicator of COVID‐19 disease mortality?. Journal of Medical Virology.
2. Gandini O, et al. Serum Ferritin as an independent risk factor for Acute Respiratory Distress Syndrome in COVID-19 Patients in Rome Italy. The Journal of Infection (2020).
3. Dahan S, et al. Ferritin as a Marker of Severity in COVID-19 Patients: A Fatal Correlation. The Israel Medical Association journal: IMAJ 8.22 (2020).
4. Lin Z, et al. Serum ferritin as an independent risk factor for severity in COVID-19 patients. Journal of Infection (2020).
5. Bataille S, et al. Could ferritin help the screening for COVID-19 in hemodialysis patients? Kidney International (2020).
6. Ruscitti P, et al. Ferritin and Severe COVID-19, from Clinical Observations to Pathogenic Implications and Therapeutic Perspectives. The Israel Medical Association journal: IMAJ 8.22 (2020).
7. Gómez-Pastora J, et al. Hyperferritinemia in critically ill COVID-19 patients–Is ferritin the product of inflammation or a pathogenic mediator?. Clinica Chimica Acta; International Journal of Clinical Chemistry (2020).
8. Fauter M, et al. Low glycosylated ferritin is a sensitive biomarker of severe COVID-19. Cellular & Molecular Immunology (2020): 1-3.