With no way to identify Omicron and Delta patients, doctors struggle with treatment decisions.

Most U.S. doctors have no way to determine which variant of the coronavirus a patient is carrying, a distinction that could mean the difference between life and death.

High-risk patients carrying the Delta variant could benefit greatly from two particular monoclonal antibody treatments shown to reduce hospitalization and death. But those medications would most likely do nothing for patients with Omicron, who would only respond to a third antibody treatment that is in very short supply.

While U.S. officials have endorsed using a workaround test that can identify Omicron’s genetic signature, experts say it’s not feasible for large health systems facing a crush of patients to employ in each case.

That makes treating patients challenging in places like Maryland, where cases are spiking and Omicron accounts for roughly 58 percent of them. The Delta variant is also holding strong in the Great Plains and swaths of the West, including California.

While there is no approved test to determine each individual’s variant, a national network of state and other labs use genome-sequencing tests to track variants broadly in communities. Health systems then use those regional estimates or their own data to decide which antibody treatments to use in their clinics and hospitals.

Many of them concluded that a community of largely Delta patients would benefit most from the antibody drugs made by Regeneron and Eli Lilly, while communities where Omicron patients are predominant would benefit from antibodies from GlaxoSmithKline and Vir Biotechnology.

Federal officials have dabbled with making the decision for the nation. On Dec. 23, they stopped shipments of antibody treatments by Eli Lilly and Regeneron after the Centers for Disease Control and Prevention said 73 percent of U.S. Covid cases were Omicron.

An outcry followed from Republican political leaders, who argued that some people in their states were still infected with Delta. And on Tuesday, the C.D.C. slashed its estimate of national Omicron cases to 59 percent. On Dec. 31, federal officials resumed national shipping all of the antibody treatments.

For the next few weeks, as the country grapples with this uneven mix of both variants, tailoring treatments to each patient will be “extraordinarily difficult,” said Dr. Alex Greninger, assistant director of the clinical virology laboratories at the University of Washington Medical Center.

Dr. Greninger is credited with developing one of the first tests to detect the coronavirus in the United States. But he is pessimistic that health systems can pivot quickly to sort out which patients have Delta or Omicron. And although a shortcut test can detect Omicron, there’s no simple way to report the results in bulk, he said.

What’s more, the genome sequencing used by public health officials takes nearly a week — too long to target the early antibody treatments that have been found to reduce the need for hospitalizations. That makes patient care particularly difficult right now, said Dr. Mark Siedner, an infectious disease clinician and researcher at Massachusetts General Hospital.

In Massachusetts and nearby states, an estimated 44.5 percent of cases are Omicron. Dr. Siedner said his health system has stopped using the Regeneron and Eli Lilly antibodies that are not effective against Omicron and are “anxiously awaiting” more doses of the effective treatment by GlaxoSmithKline and Vir Biotechnology.

“We’re in a holding pattern and it’s a terrible time to be in that place,” he said.