| Category | Definition |
| Localised | Upper and/or lower respiratory tract disease without any other systemic involvement or constitutional symptoms |
| Early systemic | Any, without organ-threatening or life-threatening disease |
| Generalised | Renal or other organ threatening disease, serum creatinine <500 μmol/litre (5.6 mg/dl) |
| Severe | Renal or other vital organ failure, serum creatinine >500 μmol/litre (5.6 mg/dl) |
| Refractory | Progressive disease unresponsive to glucocorticoids and cyclophosphamide |