- Spurious: platelet clumping, mixed cryoglobulinemia
- Primary (no other evident associated disease)
- Secondary
- Infection or inflammation from any number of causes
- Medications
- Glucocorticoids
- G-CSF
- Catecholamines (epinephrine)
- Lithium
- All-trans retinoic acid
- Case reports of other drugs
- Hyposplenism
- Cigarette smoking
- Stress (physical or emotional stress, vigorous exercise, heatstroke)