What constitutes a fever? States, companies write their own rules for temperature screening in a pandemic.

COVID-19 screening guidelines in Georgia, Ohio and Pennsylvania suggest that workers with temperatures of at least 100.4 degrees should be sent home because they could be infected with the novel coronavirus.

But the cutoff is 100 degrees in Texas. And even lower in Minnesota and Delaware: 99.5 degrees.

Some states don't suggest temperature screenings at all.

As states slowly start to reopen after weeks of coronavirus shutdowns, companies and workers face a patchwork of safety guidelines about what temperature should be a COVID-19 warning sign.

Without a national screening strategy, states and businesses are creating their own guidelines and establishing informal disease-surveillance systems with different detection levels, potentially creating confusion about what is a safe temperature.

At Walmart, a temperature above 100 degrees is enough for a worker to be sent home. At Amazon, it's 100.4.

"Where do you set the bar? Well, what is your threshold for risk?" said Steven Lawrence, an infectious disease specialist and professor at Washington University in St. Louis. "I think it would make it easier if there was one standard temperature we were using."

Fever is one of several symptoms of COVID-19, the disease caused by the coronavirus, along with a list that includes shortness of breath, coughing, loss of taste and smell, sore throat and headache.

But fever is the one symptom that can be measured and checked externally — an especially important distinction given the scarcity of rapid diagnostic testing.

Fever screening — used in previous disease outbreaks, including severe acute respiratory syndrome (SARS) and Ebola — has become common during the coronavirus pandemic but is far from perfect. Though casting a wide net by setting a relatively low temperature threshold will catch more COVID-19 cases, it also will snag many people who have nothing more than a common cold.

A fever also can be a weak signal of COVID-19, absent in the estimated 20 percent of COVID-19 cases that exhibit no symptoms at all. A recent study in the New England Journal of Medicine from Wuhan, China, where the worldwide outbreak originated, found that only 44 percent of people admitted to the hospital for COVID-19 came in with a temperature of at least 99.5 degrees.

"So you're missing half the cases walking in the door," said David Thomas, director of the Division of Infectious Diseases at Johns Hopkins University School of Medicine, who cited the journal study as a sign of the screening problems.

Another challenge is the lack of a fever definition.

"There's no binary absolute value for a fever," Thomas said.

The medical study in Wuhan defined it as at least 99.5 degrees.

A layperson's definition would probably start at 100 — well above a normal temperature of about 98.6 degrees.

Many doctors consider a fever to be 100.4 degrees Fahrenheit, which is 38 degrees Celsius. Body temperature also can fluctuate during the day and change depending on how it is measured.

Even the Centers for Disease Control and Prevention doesn't give a clear definition. It describes "fever" as a COVID-19 symptom but doesn't provide a temperature. Its rules for reportable illnesses include a fever defined as a temperature of at least 100.4 degrees or just someone who "feels warm to the touch." Its influenza surveillance network, which detected COVID-19 infections early on, reports cases with temperatures starting at 100 degrees. The CDC's guidelines for employees who may have been exposed to COVID-19 recommend screening to ensure "the employee doesn't have a temperature or symptoms," yet they don't define what that temperature is.

The CDC did not respond to a request for comment.

The White House's "Opening Up America Again" plan also recommends that employers conduct "temperature checks" but does not elaborate.

Some states and businesses have taken to defining fever on their own, arriving at a bewildering range of temperatures to be taken from different people and resulting in different outcomes.

Tennessee asks restaurants to screen workers and customers with a no-touch thermometer and to bar anyone with a reading of at least 100.4 degrees.

Ohio suggests workers stay home if they have a fever of at least 100.4.

Georgia also uses 100.4 degrees. Gyms, fitness centers and day-care centers need to fever screen not just workers, but also customers; restaurants only need to post signs warning people with fevers not to come in.

Connecticut recommends employees take their own temperatures and stay home if it is at least 100.4 degrees. Iowa has similar recommendations.

Maryland has not issued fever-screening guidelines. Virginia suggests that businesses ask workers to take their temperatures and tell them whether they have a fever of at least 100.4 degrees. The District of Columbia recommends that convenience stores and supermarkets ask workers whether they have a fever.

Texas recommends that businesses screen workers for temperatures of at least 100 degrees and ask them not to return until three days after the fever goes away.

Delaware settled on one of the strictest standards in the country. It requires essential businesses such as doctor's offices and nursing homes to bar workers with temperatures of at least 99.5 degrees. That triggers a review process that could include an at-home quarantine.

Delaware's public health officials chose that relatively low threshold after reviewing COVID-19 medical studies, Stacey Hofmann of the state's Joint Information Center said in a statement.

"By selecting 99.5 instead of 100.4," Hofmann said, "we are able to more effectively screen (increase sensitivity) in order to help identify those who may be at risk."

In Florida, a governor's task force recently suggested that businesses monitor workers' temperatures, but that had not yet been incorporated into the state's plans, according to the governor's office.

Colorado until recently required companies to do daily employee fever checks and scrutinize anyone who posted a temperature of at least 99.2 degrees. Now, employees only need to be sent home if they have a fever of at least 100.4 degrees.

The change was made to follow CDC guidance, according to a Colorado government statement.

Innumerable state guidelines have led companies to seek help understanding what they should be doing, said Daniel Kaplan, an attorney with Foley & Lardner in Milwaukee, who works with companies seeking to reopen.

"We're seeing differences among the states that aren't necessarily explicable," Kaplan said.

Although temperature screening might not stop many COVID-19 infections, they do serve another purpose, said Lawrence of Washington University in St. Louis.

“It’s a visible and noticeable intervention to show the workforce and the public that they are taking this seriously,” he said.