Final answer:
A cerebrospinal fluid (CSF) test is a definitive test that can be ordered to diagnose a subarachnoid hemorrhage when imaging results are negative for blood.
Step-by-step explanation:
A definitive test that can be ordered in the case of negative imaging for blood but a strong belief in a subarachnoid hemorrhage is a cerebrospinal fluid (CSF) test.
CSF can be obtained through a lumbar puncture procedure, which involves withdrawing a small amount of CSF from the subarachnoid space.
The CSF test can help diagnose various neurological diseases, including subarachnoid hemorrhage, by analyzing the cells, protein, glucose levels, and performing a culture examination for microorganisms.