question: How can we measure blood oxygen saturation noninvasively but not blood glucose?
comment: Blood oxygen saturation is very easy to measure. You're looking at two *very* specific wavelengths of light, which corresponds to the wavelength at which the haemoglobin molecule absorbs light either with or without a bonded oxygen. Compare the two and calculate. Since glucose contains a whole bunch of bonds (C-O, O-H, C-H) which are everywhere in your body, specifically identifying glucose by its absorption doesn't work.
passages: Glucose levels in a blood sample taken from the vein (called a blood plasma value) may differ a little from glucose levels obtained with the more common finger stick. If a venous sample is desired, results may be ready in 1 to 2 hours or longer. 
  
  In case a venous sample is preferred, a health professional will take the blood sample. The professional taking a venous blood draw will typically include the following procedures: 
   The market introduction of noninvasive blood glucose measurement by spectroscopic measurement methods, in the field of near-infrared (NIR), by extracorporal measuring devices, has not been successful because the devices measure tissue sugar in body tissues and not the blood sugar in blood fluid. To determine blood glucose, the measuring beam of infrared light, for example, has to penetrate the tissue for measurement of blood glucose.
   Blood glucose levels discussed in this article are venous plasma or serum levels measured by standard, automated glucose oxidase methods used in medical laboratories. For clinical purposes, plasma and serum levels are similar enough to be interchangeable. Arterial plasma or serum levels are slightly higher than venous levels, and capillary levels are typically in between. This difference between arterial and venous levels is small in the fasting state but is amplified and can be greater than 10% in the postprandial state. On the other hand, whole blood glucose levels (e.g., by fingerprick meters) are about 10–15% lower than venous plasma levels. Furthermore, available fingerstick glucose meters are only warranted to be accurate to within 15% of a simultaneous laboratory value under optimal conditions, and home use in the investigation of hypoglycemia is fraught with misleading low numbers. In other words, a meter glucose reading of 39 mg/dL could be properly obtained from a person whose laboratory serum glucose was 53 mg/dL; even wider variations can occur with "real world" home use.
   Glucose is measured in whole blood, plasma or serum. Historically, blood glucose values were given in terms of whole blood, but most laboratories now measure and report plasma or serum glucose levels. Because red blood cells (erythrocytes) have a higher concentration of protein (e.g., hemoglobin) than serum, serum has a higher water content and consequently more dissolved glucose than does whole blood. To convert from whole-blood glucose, multiplication by 1.14 has been shown to generally give the serum/plasma level
   Glucose concentrations do not necessarily have to be measured in blood vessels, but may also be determined in the interstitial fluid, where the same levels prevail – with a time lag of a few minutes – due to its connection with the capillary system. However, the enzymatic glucose detection scheme used in single-use test strips is not directly suitable for implants. One main problem is caused by the varying supply of oxygen, by which glucose is converted to glucono lactone and HO by glucose oxidase. Since the implantation of a sensor into the body is accompanied by growth of encapsulation tissue, the diffusion of oxygen to the reaction zone is continuously diminished. This decreasing oxygen availability causes the sensor reading to drift, requiring frequent re-calibration using finger-sticks and test strips.
   As of 2010, Heather Clark of Draper Laboratory was developing a method to measure blood glucose concentration without finger-pricking. The method uses a nano-sensor, like a miniature tattoo, just several millimeters across, that patients apply to the skin. The sensor uses near-infrared or visible light ranges to determine glucose concentrations. Normally to regulate their blood glucose levels, diabetics must measure their blood glucose at several times a day by taking a drop of blood obtained by a pinprick and inserting the sample into a machine that can measure glucose level. The nano-sensor approach would supplant this process.
  
  Section::::Notable innovations.
   Two major methods have been used to measure glucose. The first, still in use in some places, is a chemical method exploiting the nonspecific reducing property of glucose in a reaction with an indicator substance that changes color when reduced. Since other blood compounds also have reducing properties (e.g., urea, which can be abnormally high in uremic patients), this technique can produce erroneous readings in some situations (5–15 mg/dL has been reported). The more recent technique, using enzymes specific to glucose, is less susceptible to this kind of error. The two most common employed enzymes are glucose oxidase and hexokinase. Average blood glucose concentrations can also be measured. This method measures the level of glycated hemoglobin, which is representative of the average blood glucose levels over the last, approximately, 120 days.
   Section::::Non-invasive technologies.
  
  Some new technologies to monitor blood glucose levels will not require access to blood to read the glucose level. Non-invasive technologies include microwave/RF sensing, near IR detection, ultrasound and dielectric spectroscopy. These may free the person with diabetes from finger sticks to supply the drop of blood for blood glucose analysis.
   Section::::Glucose measurement.
  
  Section::::Glucose measurement.:Sample source.
  
  Glucose testing in a fasting individual, show comparable levels of glucose in arterial, venous, and capillary blood. But following meals, capillary and arterial blood glucose levels can be significantly higher than venous levels. Although these differences vary widely, one study found that following the consumption of 50 grams of glucose, "the mean capillary blood glucose concentration is higher than the mean venous blood glucose concentration by 35%."
  
  Section::::Glucose measurement.:Sample type.
   Blood glucose monitoring
  
  Blood glucose monitoring is a way of testing the concentration of glucose in the blood (glycemia). Particularly important in diabetes management, a blood glucose test is typically performed by piercing the skin (typically, on the finger) to draw blood, then applying the blood to a chemically active disposable 'test-strip'. Different manufacturers use different technology, but most systems measure an electrical characteristic, and use this to determine the glucose level in the blood. The test is usually referred to as capillary blood glucose.
   Long-term control of blood sugar concentration can be measured by the concentration of Hb A. Measuring it directly would require many samples because blood sugar levels vary widely through the day. Hb A is the product of the irreversible reaction of hemoglobin A with glucose. A higher glucose concentration results in more Hb A. Because the reaction is slow, the Hb A proportion represents glucose level in blood averaged over the half-life of red blood cells, is typically 50–55 days. An Hb A proportion of 6.0% or less show good long-term glucose control, while values above 7.0% are elevated. This test is especially useful for diabetics.
   The blood sugar content of a healthy person in the short-time fasting state, e.g. after overnight fasting, is about 70 to 100 mg/dl of blood (4 to 5.5 mM). In blood plasma, the measured values are about 10-15 % higher. In addition, the values in the arterial blood are higher than the concentrations in the venous blood since glucose is absorbed into the tissue during the passage of the capillary bed. Also in the capillary blood, which is often used for blood sugar determination, the values are sometimes higher than in the venous blood. The glucose content of the blood is regulated by the hormones insulin, incretin and glucagon. Insulin lowers the glucose level, glucagon increases it. Furthermore, the hormones adrenaline, thyroxine, glucocorticoids, somatotropin and adrenocorticotropin lead to an increase in the glucose level. In addition, there is also a hormone-independent regulation, which is referred to as glucose autoregulation. After food intake the blood sugar concentration increases. Values over 180 mg/dl in venous whole blood are pathological and are termed hyperglycemia, values below 40 mg/dl are termed hypoglycaemia. When needed, glucose is released into the bloodstream by glucose-6-phosphatase from glucose-6-phosphate originating from liver and kidney glycogen, thereby regulating the homeostasis of blood glucose concentration. In ruminants, the blood glucose concentration is lower (60 mg/dL in cattle and 40 mg/dL in sheep), because the carbohydrates are converted more by their gut flora into short-chain fatty acids.
   Two other factors significantly affect glucose measurement: hematocrit and delay after blood drawing. The disparity between venous and whole blood concentrations is greater when the hematocrit is high, as in newborn infants, or adults with polycythemia. High neonatal hematocrits are particularly likely to confound glucose measurement by meter. Second, unless the specimen is drawn into a fluoride tube or processed immediately to separate the serum or plasma from the cells, the measurable glucose will be gradually lowered by "in vitro" metabolism of the glucose at a rate of approximately 7 mg/dL/h, or even more in the presence of leukocytosis. The delay that occurs when blood is drawn at a satellite site and transported to a central laboratory hours later for routine processing is a common cause of mildly low glucose levels in general chemistry panels.
   Most of the non-invasive methods under development are continuous glucose monitoring methods and offer the advantage of providing additional information to the subject between the conventional finger stick, blood glucose measurements and over time periods where no finger stick measurements are available (i.e. while the subject is sleeping).
  
  Section::::Effectiveness.
   To monitor the body's response to blood glucose-lowering therapy, glucose levels can be measured. Blood glucose monitoring can be performed by multiple methods, such as the fasting glucose test which measures the level of glucose in the blood after 8 hours of fasting. Another test is the 2-hour glucose tolerance test (GTT) – for this test, the person has a fasting glucose test done, then drinks a 75-gram glucose drink and is retested. This test measures the ability of the person's body to process glucose. Over time the blood glucose levels should decrease as insulin allows it to be taken up by cells and exit the blood stream.
   The lag time discussed above has been reported to be about 5 minutes. Anecdotally, some users of the various systems report lag times of up to 10–15 minutes. This lag time is insignificant when blood sugar levels are relatively consistent. However, blood sugar levels, when changing rapidly, may read in the normal range on a CGM system while in reality the patient is already experiencing symptoms of an out-of-range blood glucose value and may require treatment. Patients using CGM are therefore advised to consider both the absolute value of the blood glucose level given by the system as well as any trend in the blood glucose levels. For example, a patient using CGM with a blood glucose of 100 mg/dl on their CGM system might take no action if their blood glucose has been consistent for several readings, while a patient with the same blood glucose level but whose blood glucose has been dropping steeply in a short period of time might be advised to perform a fingerstick test to check for hypoglycemia.
   The principles, history and recent developments of operation of electrochemical glucose biosensors are discussed in a chemical review by Joseph Wang.
  
  Section::::Glucose sensing bio-implants.
   One approach to achieving long-term glucose sensing is to measure and compensate for the changing local oxygen concentration. Other approaches replace the troublesome glucose oxidase reaction with a reversible sensing reaction, known as an affinity assay. This scheme was originally put forward by Schultz & Sims in 1978. A number of different affinity assays have been investigated, with fluorescent assays proving most common. MEMS technology has recently allowed for smaller and more convenient alternatives to fluorescent detection, via measurement of viscosity. Investigation of affinity-based sensors has shown that encapsulation by body tissue does not cause a drift of the sensor signal, but only a time lag of the signal compared to the direct measurement in blood.
   Fehling's solution was used for many years as a diagnostic test for diabetes, a disease in which blood glucose levels are dangerously elevated by a failure to produce enough insulin (type 1 diabetes) or by an inability to respond to insulin (type 2 diabetes). By measuring the amount of oxidizing agent (in this case, Fehling's solution) reduced by glucose, determining the concentration of glucose in the blood or in the urine is possible. This then enables the right amount of insulin to be injected to bring blood glucose levels back into the normal range.
  
  Section::::Importance in food chemistry.
   Investigations on the use of test strips have shown that the required self-injury acts as a psychological barrier restraining the patients from sufficient glucose control. As a result, secondary diseases are caused by excessive glucose levels. A significant improvement of diabetes therapy might be achieved with an implantable sensor that would continuously monitor blood sugar levels within the body and transmit the measured data outside. The burden of regular blood testing would be taken from the patient, who would instead follow the course of their glucose levels on an intelligent device like a laptop or a smart phone.
   Glucose test
  
  Many types of glucose tests exist and they can be used to estimate blood sugar levels at a given time or, over a longer period of time, to obtain average levels or to see how fast body is able to normalize changed glucose levels. Eating food for example leads to elevated blood sugar levels. In healthy people these levels quickly return to normal via increased cellular glucose uptake which is primarily mediated by increase in blood insulin levels.
   The same principle is used in test strips that have been commercialized for the detection of diabetic ketoacidosis (DKA). These test strips use a beta-hydroxybutyrate-dehydrogenase enzyme instead of a glucose oxidizing enzyme and have been used to detect and help treat some of the complications that can result from prolonged hyperglycemia.
  
  Blood alcohol sensors using the same approach, but with alcohol dehydrogenase enzymes, have been tried and patented but have not yet been successfully commercially developed.
  
  Section::::Meter use for hypoglycemia.
   The diagnosis criteria stated above by the World Health Organization (WHO) are for venous samples only (a blood sample taken from a vein in the arm). An increasingly popular method for measuring blood glucose is to sample capillary or finger-prick blood, which is less invasive, more convenient for the patient and requires minimal training to conduct. Though fasting blood glucose levels have been shown to be similar in both capillary and venous samples, postprandial blood glucose levels (those measured after a meal) can vary. The diagnosis criteria issued by the WHO are only suitable for venous blood samples. Given the increasing popularity of capillary testing, the WHO has recommended that a conversion factor between the two sample types be calculated, but no conversion factor had been issued by the WHO, despite some medical professionals adopting their own.
   Traditional fingerprick testing of blood glucose levels measures the level at a single point in time. CGM use allows trends in blood glucose to be displayed over time. Users must calibrate CGM devices with traditional blood glucose measurements. 
   To prevent contamination of the sample with intravenous fluids, particular care should be given to drawing blood samples from the arm opposite the one in which an intravenous line is inserted. Alternatively, blood can be drawn from the same arm with an IV line after the IV has been turned off for at least 5 minutes, and the arm has been elevated to drain infused fluids away from the vein. Inattention can lead to large errors, since as little as 10% contamination with a 5% glucose solution (D5W) will elevate glucose in a sample by 500 mg/dL or more. The actual concentration of glucose in blood is very low, even in the hyperglycemic.
  
presuppositions: Blood oxygen saturation and blood glucose are both measured in a way that could be done noninvasively. 
corrections: Blood oxygen saturation requires looking at absorption levels, where as blood glucose is not measured in the same way. 
raw_presuppositions: Blood oxygen saturation and blood glucose are both measured in a way that could be done noninvasively.  Blood oxygen saturation is as straightforward as blood glucose to measure.
raw_corrections: Blood oxygen saturation requires looking at absorption levels, where as blood glucose is not measured in the same way.  Glucose contains a lot of bonds that are hard to measure.