You're feeling sick, so you go to the doctor, who orders blood tests to help figure out what's ailing you. A phlebotomist (fancy word for "person who draws blood") pokes a needle in your arm, pops a test tube onto the other end of the needle, and the tube fills with blood.

Depending on which tests the doctor has ordered, and how many, the phlebotomist might draw two or three tubes full. They stick a cotton ball on the wound and ask you to hold it there while they wrap a bandage around your arm to hold it in place more permanently, and send you on your way without even so much as a cookie or a glass of orange juice. (You only get that when you donate blood for transfusion.) A few days later your doctor's assistant calls to tell you the results.

Ever wonder what happens in between? And how a tiny test tube of blood can reveal so much about your health?

My wife, Kathy, works in a medical lab. With her assistance, I will tell you what happens to your blood sample, and how she and the other medical technologists like her can tease out the most esoteric details from the body's most vital fluid.

Let's start at the beginning. The phlebotomist pokes the needle in your arm, then attaches a test tube to the needle. Why does the blood flow into the test tube? It's not your blood pressure, or blood would have been squirting out the open end of the needle the moment the other end hit the vein. In the old days, blood was drawn into syringes, and the phlebotomist had to pull back on the plunger to create a vacuum that sucked the blood into the syringe. (Okay, technically, vacuum doesn't suck. It lowers the back pressure, so everything in contact with it pushes harder toward the vacuum. So in a sense, it is your blood pressure that does the pushing; it's just not enough when there's air pushing back.) That's how the test tube does it, too. There's a partial vacuum inside, which sucks blood into the tube until the pressure inside the tube is the same as that inside your vein. It's just enough vacuum to fill the tube to the desired level, which varies depending on how much blood will be needed for testing. You may notice that the rubber stopper at the top of the tube is colored. Each color corresponds to a particular type of preservative, or in the case of red or yellow tops, no preservative at all. Lavender tubes have EDTA in them, blue tubes have sodium citrate, and green tubes have heparin. The reason for different preservatives is because each one interferes with certain tests. EDTA and heparin mess with coagulation, so sodium citrate is used for coagulation studies. Sodium citrate and EDTA mess with blood chemistry, so Heparin is used for chemistry studies. Heparin messes with the structure of the red cells, so it's not good for morphology testing. And for serology testing (antibodies), you want the blood to clot first, so you use a red-top tube. There's still one instance when you'll see the phlebotomist use a syringe: when they're drawing blood from an artery. This is generally done to measure the dissolved gas content of the blood (oxygen, carbon monoxide, carbon dioxide, bicarbonates) and its pH (acidity/alkalinity). That testing has to be done within about 30 minutes, so the blood is taken directly from your arm to the blood gas instrument, which runs the blood past electrodes that have gas-permeable membranes that allow the various components it's looking for to diffuse through and be measured. Test-tube samples generally take a longer route to the lab.

Doctor's offices seldom have their own laboratories anymore. Like everything else in our modern society, smaller laboratories keep getting gobbled up by bigger laboratories until there's usually just one or two labs per city nowadays. Sometimes your doctor or hospital will have a contract with a lab that ships samples clear out of state to be processed in a mega-factory type environment. It can take up to a day to get there. That's one reason why those tubes have preservatives in them.

Whether it's near or far, however, the same thing happens when your blood arrives: your sample goes into triage. "Stat" tests, which the doctor wants the results of immediately, get highest priority. Routine testing is set aside until the stats are taken care of. And the occasional sample is rejected, usually for being drawn in the wrong type tube for the test that's ordered, or insufficient volume, hemolysis, clotting, or age (of the sample, not you!)

Once the sample has been triaged, it's divided up into "aliquots." That's a fancy word for "portions." There may be several tests ordered on the same sample, and rather than send the sample around to each work station one at a time, it's divided up at the processing bench into multiple samples, one for each type of test. If multiple tests are done on one instrument, only one sample is required for those.

The most common test is called the Complete Blood Count (CBC), and is just what you would expect from the name: a count of all the various cell types in a standard volume of blood. Human blood has three main ingredients: small red cells that carry oxygen, much larger white cells that fight infection, and teeny-tiny platelets that assist in clotting. In a healthy person's blood, there are far more red cells (millions per cubic centimeter) than the others. White cells number in the thousands per cc; platelets in the hundreds. When those numbers are out of whack, so is the donor's health.

In the old days, medical technologists (never call them "technicians"!) would look at these blood samples through a microscope and manually count every cell within a calibrated volume on a microscope slide called a hemacytometer. Nowadays, most CBCs are done by machines that watch blood cells stream past a sensor. The machines can distinguish platelets from red cells from white cells, and even distinguish different kinds of white cells...most of the time.

Low red counts indicate anemia. Low platelet counts can mean internal clotting, a bone marrow problem, or that the patient is on chemotherapy, among other things. But the most common deviation from the norm is a high white count. That indicates that there's an infection somewhere in the body, and the circulatory system is fighting that infection by creating more white cells. White cells come in different types, so the ratio of lymphocytes, monocytes, segmented neutrophils, eosinophils, and basophils, indicates what type of infection you're dealing with. For instance, a high seg count indicates a bacterial infection, whereas a high lymph count indicates a viral infection. (It can also indicate leukemia. A skilled tech can tell which it is by the way the cells look.)

This differentiation between white cell types is called, not surprisingly, a Differential, or Diff for short. About one sample in twenty is unusual enough that the machine can't figure it out, in which case the tech has to do a manual diff. That's still done under a microscope. A drop of blood is smeared out into a wide fan on a microscope slide, stained so the white cells will stand out (they turn purple!) and the tech studies the smear at about 500x, counting every white cell they see in each field of view until they come up with 100 white cells. This often takes 15-20 fields. The tech keeps track of how many of each type of cell they see, and they report the ratio to the doctor just as the machine would.

There are two other parameters in a CBC: the hematocrit and hemoglobin. This is a measure of how thick the blood is (hematocrit) and how much of the oxygen-carrying molecule (hemoglobin) it contains.

The next most common thing that blood is tested for is chemistry. Blood contains dozens of various chemical components, each of which reveals something about the way your body functions. Doctors seldom order a single chemistry test; they order a "panel" of tests. A basic, very common panel would be a Comprehensive Metabolic Panel (CMP) that consists of: electrolytes (sodium, potassium, chloride, and CO2), glucose, kidney function (creatinine and BUN), and hepatic functions (liver enzymes).

Most chemistry tests are performed on blood serum, which is the liquid left behind after blood has clotted and has been spun down in a centrifuge until the solid material has settled to the bottom of a test tube. This is different from plasma, which is what you get when blood hasn't clotted, but has been spun down to separate the cells from the liquid. Many chemistry tests can be performed on plasma, too.

Again, in the old days, lab techs would suck up measured amounts of the sample in a pipette (often by mouth, which nowadays would get you thrown out of a lab so fast your lab coat would flap behind you), and that measured amount of serum would be added to reagents in a test tube. Some of those reagents would be "indicators," and the sample would be studied for color changes, which would reveal the type of chemical reaction occurring inside the tube.

Nowadays, the machines do a similar job, but it's all automated. This "spectrophotometry" as it's called is done in miniature test tubes called "cuvettes" that are lined up by the dozen inside the machine and filled with reagents from an assembly line of reagent and diluent reservoirs, then read by photometers. If you remember your Monty Python, this is the machine that goes "ping," the most expensive machine in the entire laboratory.

Some chemistries are tested with electrophoresis, which differs from spectrophotometry in that the machine is testing the rate of movement of various molecules when exposed to an electrical potential. Small molecules move faster than large ones, so the molecular weight of various components in a sample can be measured by how far they move through the test gel. Each of the substances being tested for has a known molecular weight, so the amount of each in a sample can be determined by simply measuring how much of it has traveled the correct distance in the allotted time.

Another part of the lab deals with coagulation studies. This, too, is just what it sounds like. Blood is studied to see how well it clots, and whether it clots when it's supposed to, and whether the medicine that's supposed to regulate your clotting factors is working effectively.

The most common tests here are called "Protimes" and "Partial Thrombin Times" (PTTs). The Protime measures the extrinsic clotting pathway, which is the way blood clots when the body is injured enough for blood to escape, and the PTT measures the intrinsic clotting pathway, which is the way blood clots when the trigger comes from damage within the body.

In order for blood to clot, a lot of things have to go right. There's a cascade of thirteen factors that all work in sequence, and various illnesses and medications can knock out various factors, inhibiting the blood's ability to clot. This is normally not a good thing, but if a person is prone to strokes or heart attacks (which are often caused by clots), their odds of survival go up if their blood's ability to clot is reduced. This is a delicate balance, though, so people on blood-thinning medication need frequent testing to make sure their blood can still clot when it should.

As practically any job applicant or athlete knows, labs can also figure out what other drugs you've been taking, and how much of those drugs are still in your system. Many drugs are tested in urine samples, but some are done with blood. There's testing for illegal drugs, but far more important is testing for therapeutic drugs. Doctors (and patients!) want to know if the medication they're taking is actually doing what it's supposed to. Drug levels are measured like any other chemistry, though often in a separate part of the lab in the case of illegal drug testing (to satisfy licensing and chain-of-custody demands).

Have you had measles yet? Chicken Pox? Are you allergic to ragweed pollen? If so, you'll have antibodies in your blood serum that will react to molecules specific to each of those afflictions. If that blood serum is exposed to those molecules (called allergens because they trigger an allergic response), the antibodies will clump around the antigens, and the lab tech can see that reaction.

Medical labs can do all this stuff now, today. How about the future? What else will we be testing for in years to come?

Cancer is one big area of research. We can test for some specific cancers now, but we would love to have a general screening test that would come up positive if there was any active cancer anywhere in the body. That would be a fairly simple test if we could just identify a particular molecule that was always present in cancer and not otherwise, but we have yet to find that unique marker.

Alzheimer's disease is still hard to diagnose positively without examining the brain itself. Needless to say, that's considered an invasive test to be avoided. We're working on more benign tests, but again, we haven't found that magic molecule that says "Alzheimer's."

Genetic testing is still in its infancy, but we're making big strides in this area already. We can look for particular genes that make a person prone to breast cancer, for instance, various leukemias, colon cancer, glaucoma, diabetes, and any other type of hereditary disease (including stuff as odd as restless leg syndrome). We just need to know what genes to look for, and the list of genes we know to be connected with these hereditary diseases is increasing daily.

An ironic side note: red blood cells can't be used for genetic testing, because they have no genetic material in them. Red cells eject their nuclei when they mature, which lets them slide around inside the body's tiny capillaries and exchange oxygen and carbon dioxide more easily. Only the white cells contain a complete set of genes that can be studied.

In theory, anything that affects the body could be tested for. If you spend a day in Beijing, the molecules of smog that you breathe will undoubtedly become bound up in your body, and those could be teased back out and identified. If you became extremely fearful or excited several hours ago, the adrenaline and its metabolites could easily be tested for (perhaps offering police a way to screen true witnesses to a crime from people who just want to be quoted on TV).

Movies seldom get technology right, let alone future technology, but the 1981 movie Outland did a phenomenal job of predicting medical testing of the future. When one of the space station's miners mysteriously commits suicide, Dr. Lazarus runs his blood through the scanner and reports, "He ate dinner. Protein, carbohydrates...more carbohydrates. He didn't eat his vegetables." She then discovers the drug in his system that drove him crazy. It's not quite the magic salt shaker that Dr. McCoy uses on Star Trek, but it's a lot closer to reality. I figure we're probably only ten or twenty years away from having this one.

But even now, it's phenomenal just how much information can be teased out of a simple drop of blood.
 

Jerry Oltion has been a science nut since he was old enough to spell "curious." He has written science fiction almost as long, and has done astronomy somewhat less. He writes a regular column on amateur telescope making for Sky & Telescope magazine, and spends many, many nights a year out under the stars.