ACS is a clinical syndrome. Due to the ambiguity of ACS, we prefer a diagnosis based on the criterion of a new radiodensity on chest radiograph with respiratory symptoms. Many of the manifestations of bronchiolitis meet the requirements for ACS, as well as life-threatening respiratory failure. Common clinical features of ACS include lung findings on physical examination, such as wheezing, persistent coughing, and a drop in oxygen saturation. 

Broadly speaking, pneumonia without ACS cannot be distinguished from pneumonia with ACS. There is an SCD component to ACS that includes an increased respiratory rate, a decreased oxygen saturation, an elevated temperature, more labored breathing, and, often, new radiodensity on chest x-ray; however, we all recognize that the new radiodensity may lag behind the clinical symptoms by hours or even days. The cadence of respiratory decline is critical. A patient with rapidly declining respiratory status is treated very differently from a patient with a low oxygen saturation (eg, in the mid-90s) but is stable and interacting with family and friends. 

We must be able to identify individuals with SCD who have rapidly progressive ACS. This is a life-threatening condition in children and adults with SCD, but it is more common in adults. Rapidly progressive ACS is defined as decreased oxygen saturation requiring at least 3 L of oxygen to maintain oxygen hemoglobin saturation at 90% or higher or intubation and mechanical ventilation within 24 hours of the onset of respiratory symptoms. Laboratory features associated with rapidly progressive ACS include worsening anemia, decreased hemoglobin by 2 g/dL or more from baseline, and multiorgan failure involving the heart, kidney, liver, and central nervous system. Any child or adult with rapidly progressive ACS should be promptly transferred to the Intensive Care Unit in consideration of an exchange transfusion to satisfy critical oxygen demands.

In the context of the current pandemic, the evidence is clear that COVID-19 is a respiratory infection that can also result in systemic manifestations. Patients with SCD who contract the SARS-CoV-2 virus should be evaluated systematically and with a high suspicion of respiratory complications. Any respiratory infection in a patient with SCD should be managed proactively, given the risk of a life-threatening infection. We follow the American Society of Hematology recommendations for the management of SARS-CoV-2 in children and adults with SCD.