Blood Matters

January, the coldest season in my vicinity, turns out to be National Blood Donor Month. This designation, a legacy of the Nixon administration (see Proclamation 3952 of December 31, 1969), I learned last week.

Besides, blood’s hot.

HBO’s True Blood received an invigorating, early renewal notice last summer; a third season will come out in June. And on film 2009 witnessed a quick, hungry revisit from Twilight, among others vampire flicks. Just this month, Ethan Hawke revealed himself in Daybreakers as Hollywood’s first hematologist-protagonist.

So it seems that now’s the perfect time to talk about it –

Blood, always my favorite Aristotelian humor, comprises two elements – plasma (a hazy yellowish fluid) and cells. The plasma bathes the blood cells in a mixture of salts and proteins as they travel within the walls of blood vessels throughout the body (the circulation) and in the chambers of the heart. Plasma proteins include some hormones, enzymes, clotting factors and antibodies.

Let’s start with some basics on the cellular components of blood: white blood cells, red blood cells and platelets:

neutrophil as seen in a peripheral blood smear, Wikimedia Commons (WC)

White Blood Cells

White blood cells (WBCs), physically larger than the rest, serve as warriors against infection. These include a cast of various types, each with a distinct role in battling germs. The most familiar white cells in the “peripheral blood” – as doctors refer to fluid passing through arteries and veins – are neutrophils, lymphocytes and monocytes. Two other forms, eosinophils and basophils, emerge from the bone marrow and typically travel in lesser numbers.

scanning micrograph, red blood cells, WC, adapted NIH image

Red Blood Cells

Red blood cells (RBCs), the most abundant and usually uniform blood cells, carry and deliver oxygen throughout the body. Mature, circulating red cells are disc-like in shape, indented on each side, and lack nuclei. They’re loaded with hemoglobin, a complex, iron-laden molecule that binds oxygen and turns blood red.

When someone receives a transfusion, that’s usually a unit of packed red blood cells, concentrated red cells from which most of the donor’s white cells, platelets and plasma have been removed.

Platelets

Platelets are tiny, blood clotting cells. Like red cells, these cells circulate without nuclei, but they’re irregular in shape and sticky, loaded inside with plug-forming proteins and on their surfaces with adhesive receptors, ready to clump at the nick of a chin or a pinprick.

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Both cancer and its treatments can affect the bone marrow, where blood cells are formed. Some tumors, like leukemia and lymphoma, arise from blood cells. Other medical conditions cause blood cell problems, too. For example, chronic kidney disease causes anemia, and HIV infection leads to reduced T-lymphocyte counts.

For all these reasons, I think it’s helpful for everyone to have some understanding of blood and blood cells – any discussion of stem cells, bone marrow and transplantation presupposes some knowledge of these basics.

More to follow!

Meanwhile, if you’re searching for more blood info on the Web, I suggest these sites:

American Society of Hematology – Blood: the Vital Connection

America’s Blood Centers – What is Blood?

American Society of Clinical Oncology (Cancer.Net) – Understanding Blood Test Results

MedlinePlus – Blood and Blood Disorders

National Heart Lung and Blood Institute – Blood Diseases and Resources Information

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On Juno and Screening Test Stats

——–
“Well, well” says the convenience store clerk. “Back for another test?”

“I think the first one was defective. The plus sign looks more like a division symbol, so I remain unconvinced,” states Juno the pregnant teenager.

“Third test today, mama-bear,” notes the clerk.

Juno recluses herself and uses a do-it-yourself pregnancy test in the restroom, on film.

“What’s the prognosis … minus or plus?” asks the clerk.

…”There it is. The little pink plus sign is so unholy,” Juno responds.

She’s pregnant, clearly, and she knows she is.

(from Juno the movie*)
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Juno\’s pregnancy test
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Think of how a statistician might consider Juno’s predicament – when a testing device is useful but sometimes gives an unclear or wrong signal.

Scientists use two terms – sensitivity and specificity – among others, to assess the accuracy of diagnostic tests. In general, these terms work best for tests that provide binary sorts of outcomes – “yes” or “no” type situations. Sensitivity refers to how well a screening tool detects a condition that’s really present (pregnancy, in the teenager’s case). Specificity, by contrast, measures how well a test reports results that are truly negative.**

Juno’s readout is relatively straightforward – a pink plus sign or, not; the possibilities regarding her true condition are few.

Still, even the simplest of diagnostic tests can go wrong. Errors can arise from mistakes in the procedure (a cluttered, dirty store is hardly an ideal lab environment), from flawed reagents (the package might be old, with paper that doesn’t turn vividly pink in case of pregnancy) or from misreading results (perhaps Juno needs glasses).

Why does this matter, now?

The medical and political news are dense with statistics on mammograms; getting a handle on the costs of cancer screening requires more information than most of us have at our disposal.

Of course, breast cancer is not like pregnancy. Among other distinguishing features, it’s not a binary condition; you can’t be a little bit pregnant.  (Both are complicated, I know.)

To get to the bottom of the screening issue, we’ll have to delve deeper, still.

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*Thanks Juno, Dwight and everyone else involved in the 2007 film; details listed on IMBD.

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**I was surprised to find few accessible on-line resources on stats. For those who’d like to understand more on the matter of sensitivity and specificity, I recommend starting with a 2003 article by Tze-Wey Loong in the British Medical Journal. This journal, with a stated mission to “help doctors to make better decisions” provides open, free access to anyone who registers on-line.

I’ll offer an example here, too:

To measure the accuracy of Juno’s kit, a statistician might visit a community of 100 possibly pregnant women who used the same type of device. If 20 of the women are indeed pregnant (as confirmed by another test, like a sonogram), but only 16 of those see the pink plus sign, the sensitivity of the test would be 16/20, or 80 percent. And if, among the 80 women who aren’t due, 76 get negative results, the specificity would be 76/80, or 95 percent.

False negatives: among the 20 pregnant women 4 find negative results; the false negative rate (FN) is 4/20, or 20 percent.

False positives: among the 80 women who aren’t pregnant 4 see misleading traces of pink; the false positive (FP) rate is 4/80, or 5 percent.

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