The glucose tolerance test (GTT), also called the oral glucose tolerance test (oGTT) is a diabetes test used to diagnose type 2 diabetes and gestational diabetes mellitus (GDM). GTT can be used for both children and adults. The procedure involves the test subject drinking an oral glucose solution (OGS) after which blood will be drawn. Blood is drawn after one hour, two hours, or three hours to evaluate how their body processes glucose. However, to confirm diagnosis the test needs to be repeated using a different blood sample and on a different day. But to have a reproducible outcome and to avoid false results, strict quality criteria must be followed.

The patient’s general health condition, the use of medication, diet, smoking habit, physical activity, gastric emptying, blood sample handling and preparation, and the type of oral glucose solution used will influence the reproducibility and accuracy of GTT. There are different types of OGS used in different countries. Some are freshly prepared while others are commercially available and ready-to-use. In either case, there is no universal or standard procedure for preparation and the composition usually varies. For example, the excipients added to improve taste and smell can alter blood glucose levels after drinking the OGS or influence the secretion of insulin. Also, the rate of absorption of freshly prepared OGS compared to commercially available and ready to use OGS is not known. These concerns need to be adequately answered and addressed.

Freshly prepared oral glucose solution

Pure glucose is not very soluble in water and care must be taken when dissolving it in water. The glucose should be poured slowly while constantly stirring. Other forms of glucose such are glucose monohydrate are more soluble in water, but care must be taken to include the weight of the water in the calculation. For example, 55.0 g, 82.5 g, and 110.0 g of glucose monohydrate will contain 50 g, 75 g, and 100 g of pure glucose, respectively.  Tap water is used with no additive or conservative. OGS are mostly prepared in a 50 g, 75 g, or 100 g dose in quantities between 200 to 300 mL.

Although freshly prepared OGS offer the advantage of being cheaper, it comes with some disadvantages. Disadvantages of freshly prepared oral glucose solution include:

  • The exact volume of liquid or weight of glucose needed can be measured with inaccuracy. Additionally, the workplace needs to be draft-free to prevent glucose powder from being blown away.
  • The glucose solution needs to be stirred carefully until the glass of glucose solution is completely emptied. If this is done inefficiently, then sediments of glucose will be left stuck in the glass.
  • The environment used to prepare the OGS can present a hygiene risk especially for very busy practices.
  • It is a time-consuming process and requires trained personnel.

Lack of strict quality control criteria may result to false positive or false negative results. This can also temper with reproducibility especially when the test needs to be repeated to confirm diagnosis.

Commercially available ready-to-use oral glucose solutions

Several ready to use OGS are available for use in glucose tolerance tests. This includes:

  1. Trutol: Trutol is a glucose tolerance beverage used to quickly increase your blood sugar levels. One large clinical trial, the so-called HAPO study which provides most of the recommendations and guidelines for treating women with gestational diabetes used this solution. It is available from Thermofisher as 50 g solution (50 g / 296 mL), 75 g solution (75 g / 296 mL), or 100 g solution (100 g / 296 mL). In Australia, Trutol is mostly used.
  2. TopStar: This ready-to-use glucose solution is sold in Portugal and Norway and available in both orange and lemon flavor. It is available here in concentrations of 50 g, 75 g, or 100 g in 200 mL bottles. It’s also available in 300 mL bottles.
  3. GlucoCrush: Glucocrush is available in the US and sold in concentrations of 50 g, 75 g, or 100 g in 296 mL bottles. It is also provided by ThermoScientific.
  4. Other ready-to-use glucose solutions are Lucozade, Glucoral, Glucolemon, Glucorange, Rapilose, and Hycal. Other types are available in most countries, but it is difficult to tell what types are available in developing countries.

Ready-to-use OGS are more expensive but may offer some advantages such as:

  • Minimal handling required, less mistakes and lower burden on the personnel performing the GTT.
  • Pregnant women report a lower incidence of associated nausea compared to freshly prepared OGS

Side effects of oral glucose solution

Side effects of OGS are rare and hardly severe, but the following may be observed and should be reported to your doctor:

  1. Nausea and vomiting
  2. Loss of consciousness
  3. Confusion
  4. A light-headed feeling, like you might pass out.
  5. Fever
  6. Swelling of your hands or feet
  7. Allergic reaction with symptoms that include hives, itching/swelling (especially of the face/lips/tongue/throat), difficult breathing, and chest pain.