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Thursday, September 23, 2010

3.3a Secrets of Blood Testing - Introduction

Physician's Notebooks 3 - http://physiciansnotebook.blogspot.com - See Homepage 

Chapter 3: Blood Testing (BT)  - Update 09 Dec. 2017

Self-testing kits do blood sugar (glucose), cholesterol, and INR (International Normalized Ratio for those using the anti Vitamin K anticoagulant Coumadin aka "Warfarin"). And the percent oxygen saturation of hemoglobin in arterial blood (SaO2) is a non-invasive self test with rapid readout by Pulse Oximeter that also does pulse rate and, in some models, hemoglobin g/dL in instant digital readout.

In blood testing, the control  refers to a comparative state of normality so that the effect of unusual behavior and stress on a test is minimized. For most blood tests it is overnight fast, no acute illness and resting state. Blood test values are reported as concentration of chemical or cell in a volume of blood fluid; so a person tested should standardize his water intake before blood test to avoid dilution (gives false low value to test) or dehydration (gives false high value). To do that, take nothing by mouth after .

Blood Test Sample: Depending on test, the blood test is reported as per unit blood serum (S; the fluid of clotted blood sample; most tests) or blood plasma (P; of non clotted blood; all blood cell counts are done in P) or whole blood (WB, the blood remains non clotted with mixed cells and fluid; some self tests). The anti clot substance put in P or WB samples is heparin or calcium oxalate. In the body the blood cells are carried suspended in the flowing plasma.

What is in the Serum? The translucent fluid that is separate from the clot is not Plasma because its blood-clotting proteins are lost during the clotting. The after-clot blood fluid is the Serum. Most chemical tests are done on serum because it is not contaminated by anti clot agent that could affect chemical test result.

Most syringe-drawn blood-chemistry tests are reported in U.S.A. as weight of the chemical per unit volume of Serum (eg, “mgm of glucose per 100 cc or ml” a.k.a. mg %). But the test may also be reported by volume of Plasma and some automated tests collect tiny blood sample by finger stick in an anti clot heparin-treated capillary tube and the result must be reported per unit volume of WB. There are small differences in blood test number unit based on whether it is reported for WB (as in self test kits), for P (with added anti-clot) and S (fluid after blood clots). Usually, the concentrations of WB testing are c.10% lower than in same sample P or S. The differences between P and S are that plasma test concentrations are less than serum’s for glucose ( minus 5%), for phosphorus (–7%) and for potassium (–8.4%) and plasma’s more than serum’s for total protein (+4%). Those numbers are of borderline significance.

Interpreting Your Blood Test: It is important to understand the difference between “normal” and “healthy”. Normal means a test is within statistically average range based on a tested total population without apparent disease. But it does not tell whether or not the test result is really good for your health. The average, or ‘normal’, serum cholesterol for adult American in 1998 was 205 mg%, but in its risk for coronary heart disease, that was too high. Also, age and sex may need to be taken into account. Example is serum Fe (iron), which is normally lower for women than men. And with aging over 70, your body normally tolerates, and may thrive on, lower percentage of red blood cells (RBC), which in youth is a mild anemia.

Later, I give normal values. The range rather than the central, or average value emphasizes that normality is no single number. Also, bigger is not always better. Many athletes think the higher the Red Blood Cell (RBC) count the more endurance they will have, because the RBC gives blood its oxygen-carrying capacity. Within narrow limit this is correct, but once the limit of normal is exceeded, a higher RBC count increases risk of heart failure and blood clot diseases. And in testing for a metal that is an essential nutrient – Iron (Fe), Zinc (Zn) or Copper (Cu) – elevation of concentration only slightly above normal may be an enhancer of Alzheimer's and Parkinson's disease (by metal poisoning of brain cell over years), also of diabetes.

The most accurate interpretation of a test may involve relating its affect on another test. In a 55 y/o menopausal, hormone-replacement woman, a serum estradiole (E2; ovarian female sex hormone) of 20 micro-gm/dL associated with follicular stimulating hormone (FSH) of 5 IU would be considered OK because it shows that even at that relatively low E2, the pituitary gland is not being stimulated to overproduce FSH. But in another woman the same E2 level but with an FSH of 100 is a sign of female hormone deficiency, calling perhaps for treatment with estrogen. The principle demonstrated is that low blood level of one substance, by releasing negative feedback, over-stimulates a gland to make high level of second substance. But the setting is different in each individual.

Do not depend on a single testing for diagnosis or health decision. Most tests have intrinsic error ±10%. When a test is abnormal, or in case of normal finding, when it is important in diagnosis, repeat the test.
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