Blood tests are just one of many diagnostic tools a health care provider uses to evaluate your health. Blood tests can measure everything from blood alcohol level to pregnancy to white blood cell count. Each test has different normal values, or reference ranges. Your test values, whether they fall into the normal range or not, must also be considered in the context of your symptomatology.
Because everyone is unique, there is no single "normal" value for blood tests. Instead, there is a range of values that are considered to be normal. Ninety-five per cent of the normal population will fall into that range. However, reference ranges are variable with age, gender, race and even different labs using different machines. It's always best to let your doctor interpret your blood test values.
There are hundreds of blood tests your doctor can order, and each one of them will have a different reference range. Some ranges are narrow, others are wider. According to the Merck Manual, the normal range for calcium is 8.5 to 10.5 mg/dl, but glucose is 70 to 110 mg/dl. Sometimes the reference ranges are adjusted when more sensitive testing becomes available, as is the case with thyroid stimulating hormone, or TSH, used to measure thyroid function. The medical community now has differing opinions on the upper limit of TSH value and, depending on your doctor, it may be anywhere from 2.5 to 5 mIU/L.
Complete Blood Count (CBC)
The CBC is a common blood test that measures your concentration of white and red blood cells, haemoglobin, hematocrit and platelets. It is often used to detect anaemia or infection. The Merck Manuals Online Medical Library lists the reference range for white blood cells at 4,300 to 10,800/ml, red blood cells 4.2 to 5.9 million/ml, haemoglobin 13 to 18g/dl (male) or 12 to 16g/dl (female), hematocrit 45 to 52 per cent (male) or 37 to 48 per cent (female) and platelets 150,000 to 350,000/ ml.
Basic Metabolic Panel (BMP)
The BMP checks kidney function, blood sugar level and electrolyte balance (the minerals in your blood with an electric charge). It can be used to detect kidney failure, diabetes or dehydration. Reference ranges for electrolytes are sodium 135 to 145 mEq/L, potassium 3.5 to 5 mEq/L, carbon dioxide 35 to 45 mmHg and chloride 98 to 106 mEq/L; for kidneys, blood urea nitrogen (BUN) 7 to 18 mg/dl and creatinine 0.6 to 1.2 mg/dl; and for blood sugar, fasting glucose 70 to 110 mg/dL.
Remember that your blood tests should only be interpreted by a health care provider. While it might be useful to know that your cholesterol is high (reference range less than 225 mg/dl if you are below age 40), there are many other factors to be considered in evaluating your overall state of health. You might be part of the 5 per cent of the normal population that falls outside the reference ranges.