Candidate predictors to be assessed for inclusion in models (adapted from Blakey et al5)
| Variable | Description |
| Sex | Male or female |
| Age | In years at the start of the 3-year study period |
| BMI | Last recorded, in kg/m2; categorised as underweight (<18.5), normal (18.5–24.9), overweight (25- 29.9) or obese (≥30) |
| Ethnicity | Ethnicity information if available (white, black, Asian, South Asian Caribbean etc) |
| Smoking status | Last recorded, categorised as never smoker, current smoker or ex-smoker |
| Charlson comorbidity index | Score in the baseline year, categorised as 0, 1–4, 5–9, ≥10 |
| Comorbidities* | Recorded ever or active: eczema, allergic and non-allergic rhinitis, nasal polyps, anaphylaxis diagnosis, anxiety/depression diagnosis, diabetes (type 1 or 2), GERD, cardiovascular disease, ischaemic heart disease, heart failure, psoriasis |
| Comedications | In baseline year, prescription (yes/no) for paracetamol, NSAIDs, beta-blockers, statins |
| % predicted PEF | Recorded ever, expressed as percentage of predicted normal, categorised as unknown,<60%, 61%–79% and ≥80% |
| Blood eosinophil count | Last recorded, in 109 cell/L, categorised as ≤0.4 or >0.4 |
| BTS step† | |
| Step 1 | Inhaled SABA as needed |
| Step 2 | ICS or LTRA |
| Step 3 | Add LABA to ICS or use high-dose ICS (≥400 mg/day FP equivalent) |
| Step 4 | Add LTRA/Theo to (ICS+LABA) or add LABA/LTRA/Theo to high-dose ICS |
| Step 5 | Add OCS |
| Average daily dose of SABA/ICS | Cumulative dose of SABA/ICS prescribed in baseline year, expressed in mg/day albuterol or FP equivalent and divided by 365.25 |
| Prescribed daily ICS dose | Dose of ICS prescribed at last prescription of baseline year in mg/day, FP equivalents |
| ICS medication possession ratio | ICS refill rate during the baseline year: sum of number of days per pack (number of actuations per pack/number of actuations per day)/365.25 |
| ICS device type | In baseline year: categorised as no ICS, MDI, BAI or DPI |
| Spacer use with ICS pMDI | Recorded in baseline year (yes/no) |
| Oral corticosteroid use | Any maintenance prescription for corticosteroids in baseline year (yes/no) |
| Prior asthma education | Recorded ever (yes/no) |
| Primary care consults | Number of primary care consultations, categorised as 0, 1–5, 6–12, ≥13 |
| Primary care consults for asthma | Number of primary care consultations with an asthma-related Read code |
| Antibiotics with lower respiratory consult | Number of consultations that resulted in antibiotic prescription (included to capture asthma events that may have been misclassified as LRTI) |
| Acute respiratory events | Number of events in the baseline year, defined as asthma-related hospitalisation or ED attendance or an acute course of OCS or antibiotics prescription with lower respiratory consultation |
| Acute OCS courses | Number of acute courses of OCS in baseline year, categorised as 0, 1, ≥2 |
| Acute OCS courses with lower respiratory consult | Number of OCS courses with Read code for lower respiratory consultation in baseline year, categorised as 0, 1, ≥2 |
| Antibiotics courses | Number of antibiotics prescriptions with Read code for lower respiratory consultation in baseline year, categorised as 0, 1, ≥2 |
| Hospital attendance/admission | Number of asthma-related‡ ED, inpatient and outpatient attendance/admission in baseline year (as recorded in primary care data) |
| Asthma attacks | Number of asthma-related‡ hospital ED attendance, inpatient admission or acute OCS course |
| Eosinophil count | Blood eosinophil count (cells/L) categorised into high and not high (threshold of 0.35×109 cells/L) to define high/not high eosinophil count24 |
*Comorbidity recorded ‘ever’ was defined as a diagnostic Read code during the baseline year or at any time before baseline. ‘Active’ refers to those for which a diagnosis was recorded within the baseline year and/or a previous diagnosis was accompanied by a prescription for the comorbidity within the baseline year. ‘Rhinitis’ included allergic and non-allergic rhinitis.
†Based on the British guideline on the management of asthma (October 2014) for adults and children.25
‡Any patient with a lower respiratory Read code (asthma or LRTI code).
BAI, Breath-actuated inhaler; BMI, body mass index; BTS, British Thoracic Society; DPI, dry powder inhaler; ED, emergency department; FP, fluticasone propionate; GERD, gastro-oesophageal reflux disease; ICS, inhaled corticosteroids; LABA, long-acting beta antagonists; LRTI, lower respiratory tract infection; LTRA, leukotriene receptor antagonist; MDI, metered-dose inhaler; NSAIDs, non-steroidal anti-inflammatory drugs; OCS, oral corticosteroids; PEF, peak expiratory flow; SABA, short-acting b2 agonist; Theo, theophylline.