As part of routine care for SCD, some people are found to have low oxygen levels (≤ 88%) while sleeping, at rest, or with exercise. Testing is done with a small portable device positioned on the finger that measures oxygen levels during sleep, at rest, or following exercise. The investigators start oxygen treatment for people with low levels of oxygen. As a part of this study, the investigators will find out if any changes in cell "stickiness" occur with low oxygen levels (at rest, at night, or with exertion) and if cells become "less sticky" with oxygen treatment. Study subjects will be seen before testing and 2 months after testing. In some cases (people with low oxygen levels during testing), study subjects will have been prescribed oxygen, and the investigators will test the effects of that treatment on the stickiness of red cells.

In SCD, exertional hypoxia and nocturnal hemoglobin desaturation (NHD, or hemoglobin deoxygenation during sleep) are common, treatable, and associated with bad outcomes in children and young adults15,16. The median life-expectancy of SCD has risen dramatically in the last 40 years. One consequence of this is an expanding young adult population in whom the comorbidities are not yet fully characterized. The prevalence, clinical consequences, and treatment outcomes of exertional hypoxia and NHD are poorly described in adults with SCD. Therefore, it is important to identify and better understand any clinically significant hypoxia (during exercise or sleep or at rest) in this expanding adult population. The investigators will study whether RBC adhesion at baseline and when exposed to hypoxia in vitro is significantly increased in adult HbSS patients with baseline hypoxia, exertional hypoxia or nocturnal NHD due to RBC membrane changes arising from prolonged in vivo exposure to hypoxia, which may be mitigated by oxygen therapy.

Hypotheses: The investigators hypothesize that disease activity and RBC adhesion (under normoxia) will be greater in subjects with HbSS plus baseline in vivo hypoxia, exertional hypoxia, or NHD, due to RBC membrane damage from prolonged hypoxia in vivo. Successful treatment with therapeutic oxygen, at baseline, with exertion, or during sleep, may decrease RBC adhesion in vitro.

Specific Aim 1: To evaluate for resting hypoxia, exertional hypoxia or NHD, and its clinical associations, in adults with HbSS.

Specific Aim 2: To examine baseline RBC adhesion under normoxia or hypoxia in vitro in adults with HbSS, with and without in vivo resting or exertional hypoxia or NHD.

Specific Aim 3: To examine serial changes in S-RBC adhesion at baseline and with hypoxia in vitro, in adults with HbSS and resting or exertional hypoxia or NHD, before and after therapeutic intervention with oxygen.

The investigators are testing whether:

1. Subjects with Hb desaturation at baseline, with exertion, or during sleep (NHD), compared to those without, will have increased disease activity (exertional or nocturnal symptoms, priapism, WBC activation, reticulocytosis, and/or hemolysis).
2. S-RBCs from subjects with clinical Hb desaturation at rest, with exertion, or during sleep, compared to those without, will have increased adhesion at baseline and when exposed to hypoxia in vitro.

2.A. Treatment of baseline hypoxia, exertional hypoxia, and/or NHD with supplemental oxygen will decrease S-RBC adhesion and HEA, and may decrease symptoms, especially night- time symptoms.

Inclusion Criteria:

1. Male or Female > 18 year of age at the time of consent.
2. Documentation of Sickle Cell Disease, phenotypically HbSS (including S-Beta 0 thalassemia)
3. Written informed consent (and assent when applicable) obtained from subject or subject's legal representative and ability for subject to comply with the requirements of the study.
4. English speaking patient

Exclusion Criteria:

1. Ongoing Oxygen therapy.
2. active pregnancy, due to complex pathophysiology during that interval.
3. Presence of a condition or abnormality that in the opinion of the Investigator would compromise the safety of the patient or the quality of the data. -

Individuals with HbSS Sickle Cell Disease