Scientists are closing in on a long-sought goal: A blood test to screen people for Alzheimer’s disease. An experimental test did a good job of indicating how much of the telltale Alzheimer’s plaque lurks in people’s brains, Australian researchers reported Wednesday. If the test proves accurate in larger studies, it could offer a way to check people having memory problems to see who needs more definitive testing for the disease. Many blood tests are being developed and a few are used in research settings now, but only the Australian one has been validated against brain scans and other accepted diagnostic tests with good accuracy in large groups of people, said Maria Carrillo, senior director of medical and scientific relations for the Alzheimer’s Association.