Epidural catheter tests...not only the test dose

Siphon test

The catheter is held upright and a fluid level sought. If the catheter is then elevated, the fluid level should fall (see inset) as the fluid siphons in to the epidural space, which is usually under negative pressure compared with atmospheric. If the fluid column continues to rise, this may suggest subarachnoid placement. The siphon test can be reassuring, but is not mandatory.

Aspiration 

This should be considered mandatory. The Luer connector is attached to the catheter and a syringe is used to apply negative pressure. Free and continued aspiration of clear fluid can indicate subarachnoid placement of the catheter. However, if saline has been used for loss of resistance, it is not unusual for a small amount of this to be aspirated. If there is doubt, the aspirated fluid can be tested for glucose (cerebrospinal fluid will generally test positive) or mixed with thiopentone (cerebrospinal fluid forms a precipitate). If blood is freely and continuously aspirated, this suggests intravascular placement. In this case, flushing the catheter with saline and waiting before re-aspirating is a useful test. If further fresh blood is freely and continuously aspirated, this supports intravascular placement. If only blood stained saline is aspirated, it may simply indicate aspiration of blood leaking from a traumatic insertion. In either case, if there is doubt, the catheter should not be used.