First response? No lines?

I just woke up, all excitedly ran to the bathroom and did a first response test as I'm now 4 days late and that's calculated off the longest cycle I've ever been. Took the test as per usual and no lines showed up :( I'm so annoyed I live  half an hour from any shops! Guess tomorrow I will find out for sure... If I tested this afternoon will it be as accurate knowing I'm now 18dpo ?