What is a minimum effective dose?

‘Minimum effective dose’ refers to the minimum dose of ICS that maintains optimal asthma control after back titration. Minimum effective dose should be individually determined by assessment of asthma control and objective measures of lung function.

In order to be sure that a patient is using the minimum effective dose, it is necessary and reasonable to attempt reduction in the dose of ICS. This applies even when the current dose is moderate and at the risk of breakthrough symptoms (provided that the patient has a written asthma action plan and understands what to do during an exacerbation).

What is optimal asthma control?

Knowing when asthma control has been achieved is an important first step in achieving minimum effective dose.

Optimal or ideal asthma control includes:^{2, 14}

• Absent or minimal symptoms during day and night.
• Absent or minimal reliever medication (less than 3 times/week, excluding exercise).
• Normal lung function (at, or close to, best).
• No time lost from school or work due to asthma.
• Absent or minimal side effects from medication.

Optimal control also includes minimal or no asthma exacerbations,¹⁴ but these may not be totally avoidable, even in patients who are managed appropriately. In addition, the pattern of symptoms that constitutes acceptable control will depend to some extent on the dose of ICS: for example, it may be appropriate to tolerate slightly more frequent use of short-acting beta agonist (SABA) in a patient already receiving high-dose ICS (or ICS plus LABA), since a further increase in ICS dose would significantly raise the risk of adverse effects.

Role of spirometry in assessing asthma control

The concept of personal best FEV₁ (or peak flow) is useful in the assessment process, but care is needed to avoid unnecessarily high doses of ICS in an attempt to gain 'best' FEV₁. Spirometry should be regarded as just one tool in a range of tools for assessing asthma control with a view to identifying minimum effective dose. Overall, over 90% of 'best' FEV₁ is a worthwhile target. It is essential that the correct spirometry technique is used when assessing the need for dose adjustment. Spirometry should be performed and interpreted appropriately so that clinicians can be confident that the FEV₁ obtained does in fact represent the best possible result.¹⁵

How low should the dose go?

There is probably a threshold dose beneath which ICS should not be reduced, but this has not been defined and is likely to differ between individuals. It is important to consider individual susceptibility - some patients are very sensitive to low doses, while others may require a moderate dose to maintain asthma control. In patients with moderate to severe asthma, the total daily dose below which the ICS should not be reduced is likely to be in the range of 250 mcg HFA-BDP or FP, or 400 mcg BUD.³ In patients with mild asthma, the lowest effective dose is likely to be around 100 mcg HFA-BDP or FP, or 200 mcg BUD.¹⁶