The quantity (volume and concentration) of local anesthetics needed for Epidural anesthics is very large compared with that needed for Spinal anesthesia. Significant toxicity can occur if this amount is injected intrathecally or intravascularly. Safe-guards against this include the Epidural test dose. This is true whether injection is made through the needle or through an Epidural catheters.

A test dose is designed to detect both subarachnoid and intravascular injection. The classic test dose combines local anesthetic and epinephrine, typically 3 mL of 1.5% Lidocaine with 1:200,000 epinephrine(0.005 mg/mL). The 45 mg of Lidocaine, if injected intrathecally, will produce Spinal anesthesia that should be rapidly apparent. Some clinicians have suggested use of lower doses of local anesthetics, as unintended injection of 45 mg of intrathecal Lidocaine can be difficult to manage in areas such as labor rooms.

Incremental dosing is a very effective method of avoiding serious complications. If aspiration is negative, a fraction of the total intended anesthetics does is injected, typically 5 mL. This does should be large enough for mild symptoms of intravascular injection to occur but small enough to avoid seizure or cardiovascular compromise. If a clinician uses an initial test dose, is diligent about aspirating prior to each injection, and always uses incremental dosing, significant systemic toxicity or inadvertent intrathecal injections are rare.