Meanwhile, an Idaho toddler died from a kidney disease associated with E. coli infection, but health officials did not know if it was connected to contaminated spinach.
Tissue samples of the Maryland woman and some of the spinach she ate just before she became ill were submitted to state labs for testing, Washington County Health Department spokesman Rod MacRae said.
"This is a very suspicious association at this point, there's no question about it," MacRae said, adding that it could be days before test results are known. MacRae also said it is not clear that the woman's type of E. coli is the same as the one blamed on tainted spinach.
MacRae would not identify the woman beyond saying she was a Washington County resident in her 80s.
Kyle Algood, 2, of Chubbuck, Idaho, died Wednesday at Primary Children's Medical Center in Salt Lake City from hemolytic-uremic syndrome, said Dr. Christine Hahn, epidemiologist at the Idaho Department of Health.
Kyle had bloody diarrhea, and family members told hospital officials in Idaho that he had eaten packaged spinach, Hahn said.
"We know that if he had that kidney disease, it makes it very probable that he had E. coli," Hahn said.
Hahn said it would likely be next week before test results would be available.
E. coli is often spread by human or animal waste. The bacteria has killed one person and sickened at least 157 others across the country since last month.
Federal and state officials have traced the outbreak to contaminated spinach from at least one of nine farms and several processing plants in California's Monterey, San Benito and Santa Clara counties. The region produces more than half of the nation's spinach crop.