Carolyn Keller could probably coax blood from a stone. As an American Red Cross bloodmobile unit supervisor, she has to get it from only the public, and some days that can be a challenge.

According to Keller's patients, she's good at what she does.

"I didn't even feel it," said Gary Robertson, after a smiling Keller slid a 16-gauge steel needle into his vein and his 38th donated pint of blood had drained into a plastic pouch during a recent blood drive at the Potomac Electric Power Co. on Pennsylvania Avenue NW.

Keller, 46, who has worked for two years with the Red Cross Blood Services Division, performs her job with a mixture of charm, skill and a sense of purpose. Talk to her between patients at a blood collection center, and you're struck by the enormous responsibility she feels, not only for keeping donors happy and therefore eager to come back and give again, but also for screening out donors who might carry the AIDS virus, and other diseases (such as hepatitis) transmitted through blood products.

Although the Red Cross has tested all its blood for the HTLV III antibody since 1985, Keller explained that the initial interview conducted by bloodmobile personnel with each donor is very important as a first step in the screening process.

"Certain people are not acceptable," she said.

And the Red Cross makes no bones about it. A pamphlet given to each potential donor says very clearly on its cover, "If you are a man who has had sex with another man since 1977, you must not give blood or plasma."

The interview with prospective donors conducted by Keller, a former psychiatric nurse, and her colleagues gets very specific.

"Have you ever taken self-injected drugs?" (Intravenous drug injections put people at risk for contracting acquired immune deficiency syndrome.)

"Have you had night sweats, unexplained fever or weight loss, lumps in neck, arm pits or groin, discolored areas of skin or mouth, persistent cough, or persistent diarrhea?" -- all symptoms of the deadly virus.

A second warning spelled out to the donor just before giving blood is very frank.

"You are at risk for getting AIDS and spreading the AIDS virus if . . . you have been the heterosexual sex partner of a male or female prostitute within the last six months."

Donors are instructed to check a box, either "Use my blood" or "Do not use my blood," on a confidential form that is later attached to each pint.

Throughout the entire process, Keller maintains eye contact with her subject, smiling, nodding, encouraging, occasionally joking.

Part of her technique is to distract donors as she loads a lancet into the instrument she uses to prick their fingers for a required hemoglobin test. (Some people tell her it is the worst part of their bloodmobile visit.)

But another purpose of the technique is to "watch for body language," Keller said.

"People who are feeling guilty don't want to look you in the eye," she said.

Not only is the Red Cross wary of blood tainted by the AIDS virus, some people mistakenly think that the disease can somehow be transmitted to them as they're giving blood, Keller said.

"I've had well-educated people need reassurance that it {giving blood} is a safe process," Keller said. "In fact, this is the safest part of the process."

Keller said her biggest challenge is the new donor. These people may be terrified, but they also represent the potential for more blood in the future if they can be persuaded to return.

Willing donors with safe blood are critical these days to the Red Cross, which is chronically short of supplies. According to Red Cross officials, in the Washington area there is now a critical need for Type O blood, which can be used for anyone who needs blood, regardless of the recipient's type.

Keller finds herself in the position of being a salesperson for a procedure involving an instrument most of us learned to fear as toddlers, the hypodermic needle. "The first-time people usually don't want anyone to know how frightened they are," she said. "You find people so nervous, they have drenched palms. They ask, 'Is it going to hurt?' Well, yes, I'm going to put a big needle in your arm."

The hollow needle used must be relatively large, Keller explained, so as not to damage any of the blood cells as they leave the body, Keller said.

The diameter of the needle is about the same as thin spaghetti.

"But fortunately, the needles we use are very, very sharp," she added.

Bloodmobile nurses spend five weeks learning how to insert needles into veins of all sizes and degrees of toughness.

Donors who tell Keller later that the needle hasn't hurt gratify her. And she thinks that lots of them get a sense of achievement, too.

"You have to encourage people. They've got to get more out of this than packaged lemonade and a couple of cookies in the canteen."

Keller, a graduate of Boston University, chose Red Cross nursing after a two-year stint working as a psychiatric nurse in a suburban hospital.

"My husband is military. And I've done everything that can be done in nursing with the exception of O.R. {operating room}," she said.

She decided to work with the Red Cross because she wanted variety and regular hours. As a supervisor she oversees the work of five other nurses, and each day they set up in a different business, government agency or university. Some places yield more blood than others.

"The Senate is excellent," Keller said, "and Pepco has a good turnout, as do the colleges. Arnold and Porter {law firm} is very good, as are the {U.S.} Park Police, Blue Plains {sewer treatment plant} and the FBI."

Bloodmobile personnel also get to know where people are most stressed out.

"There seems to be a certain tone to some places. You see a lot of elevated blood pressures," Keller said, "while at others, like the Pentagon, where you see all those people jogging at lunchtime, there are low blood pressures, good pulses.

"We deal with some delightful people," she added. "And with some not so delightful people.

"I'm using all the knowledge as a nurse that I have in this job, plus PR skills," said Keller, turning to yet another donor. She smiles as she secures a blood pressure cuff around his upper arm.