At least 12 hours before the test? That kind of leaves a wide potential window, depending on if it's a once daily or twice daily (or more?) prescription. Can you split the tabs/capsules in half? Because maybe you could take 300mg, ~6hrs before the test... Or one pill 12 hours before will probably work. What the blood test is supposed to do is calculate lithium levels during the troughs, the lowest concentration in your blood between doses. So, 1 pill 12 hours before is probably a safe bet. But I'm really sorry that you're in this situation at all and it's totally a last resort. If there is anything else you can do, it would be worth it. If this is your last resort just make sure you drink a lot of water and do your best to eat healthy to help your body/brain/kidneys metabolize that stuff.