Where does washout collide?

In the design of crossover clinical trials, the washout is a deliberate interval placed between two treatment periods so that the first treatment clears from the body before the second begins, preventing its effect from carrying over and contaminating the estimate of the second. Willan and Pater's 1986 Carryover and the two-period crossover clinical trial analysed how an inadequate washout leaves a carryover effect that biases the within-subject comparison, and why testing for that carryover is itself problematic. The referent is a cleansing gap in time whose job is to return each subject to a common baseline, scaled in drug half-lives or days rather than any spatial quantity, and it has nothing to do with a twisted wing or a fading scan beyond the shared image of something washing out.[1]

Unit

The time between treatment periods needed to clear a prior drug's effect

Recovery means

The subject returns to baseline once the previous treatment and any carryover have cleared

Measured by

Set from drug half-life and checked by testing for a residual carryover effect between periods

In cross-sectional imaging, washout describes how quickly injected contrast medium drains out of a tissue after it has enhanced, and the rate is diagnostic: a benign adrenal adenoma takes up and then rapidly releases contrast, washing out faster than a malignant mass. Korobkin and colleagues' 1996 Delayed enhanced CT for differentiation of benign from malignant adrenal masses showed that attenuation on delayed scans, expressed as absolute or relative percentage washout, separates adenomas from metastases and carcinomas. The referent is a time-course of contrast clearance from a volume of tissue, read off serial CT attenuation values, unrelated to a trial's rest interval or a wing's twist except for the common picture of washing away.[2]

Unit

The percentage of contrast cleared from a tissue on delayed imaging

Recovery means

Not applicable

Measured by

Computed as absolute or relative washout from enhanced and delayed CT attenuation (Hounsfield) values