Sickle cell disorder	Hemoglobin composition (%)	HbA₂ level	Erythrocyte volume (MCV)	Clinical severity	Clinical features
HbSS	HbS: 80 to 95	Normal	Normal	+ + to + + + +	Severe disease
HbSS	HbF: 2 to 20	Normal	Normal	+ + to + + + +	Severe disease
HbS-β°-thalassemia	HbS: 75 to 90	Increased	Decreased	+ + to + + + +	Generally indistinguishable from SS
HbS-β°-thalassemia	HbF: 5 to 25	Increased	Decreased	+ + to + + + +	Generally indistinguishable from SS
HbS-β⁺-thalassemia	HbS: 5 to 85	Increased	Decreased	+ to + + +	Generally milder than SS
	HbA: 10 to 30
	HbF: 5 to 10
HbSS with α-thalassemia trait (−, α/ −, α)	HbS: 80 to 90	Normal	Decreased	+ + to + + + +	May be milder than SS
HbSS with α-thalassemia trait (−, α/ −, α)	HbF: 10 to 20	Normal	Decreased	+ + to + + + +	May be milder than SS
HbSC	HbS: 45 to 50	Normal	Normal	+ to + + +	Generally milder than SS; higher frequency of bone infarcts and proliferative retinal disease
	HbC: 45 to 50
	HbF: 2 to 5
HbSo Arab	HbS: 50 to 55	Normal	Normal	+ + to + + + +	Generally indistinguishable from SS
	HbO: 40 to 45
	HbF: 2 to 15
HbSD Los Angeles	HbS: 45 to 50	Normal	Normal	+ + to + + + +	May be as severe as SS
	HbD: 30 to 40
	HbF: 5 to 20
HbS/HPFH*	HbS: 65 to 80	Normal	Normal	0 to +	Usually asymptomatic
HbS/HPFH*	HbF: 15 to 30	Normal	Normal	0 to +	Usually asymptomatic
HbAS*	HbS: 32 to 45	Normal	Normal	0 to +	Asymptomatic
HbAS*	HbA: 52 to 65	Normal	Normal	0 to +	Asymptomatic