(Oral glucose tolerance test OGTT) Practical (Oral glucose tolerance test OGTT) Physiology 232 BMS Dr/Nahla Yacout 2016/2017

Objectives At the end of the lecture, students should be able to: Define OGTT, and explain its procedure Explain the normal results of the test Define Renal threshold for glucose, Tubular load, and Transport maximum Define Renal glycosuria

Oral glucose tolerance test (OGTT) OGTT measures how well your body cells are able to absorb glucose, after you ingest a given amount of it It is primarily used to diagnose Gestational diabetes Who needs OGTT?? Pregnant females are the most common, as gestational diabetes can cause pregnancy complications Recommended between 24th & 28th week of pregnancy

Procedure The patient continues to eat normal diet in the days before the test Fasts for 15 hours before the test which is generally done in the morning Urine & blood samples are collected Then the patient drinks 75gm of glucose Blood sample is then collected after 30, 60, 90 & 120 minutes Urine sample is collected after 60 & 120 minutes Glucose concentrations of the blood samples are determined & then the curve is drawn

Normal of the test Fasting ………… 80 – 110 mg% Peak level is reached in 30 – 40 minutes after glucose ingestion …………….. 130 – 140 mg% Glucose level returns to fasting value in 120 – 150 minutes Urine sample contains no glucose

In diabetic patients: Fasting blood glucose level is always above 110 mg% The peak value after glucose ingestion is reached only after 90 minutes

Renal threshold for glucose = 180 mg% Is the plasma level of glucose at which glucose first appears in the urine in more than the normal small amounts (Normally, urine contains few mg of glucose undetectable with the usual tests) Renal threshold for glucose = 180 mg%

As blood glucose level is below this level, all glucose will be reabsorbed again & no glucose will appear in urine If blood glucose level exceeds 180 mg%, all the excess glucose will be excreted in urine

Important note Glucose is a large molecule to diffuse through the cells, & so it is transported by Facilitated diffusion, which is a carrier – mediated process (These carriers are known as: Glucose transporters)

Tubular load (Filtration load) Is the amount of substance delivered to the renal tubules per minute due to process of filtration It depends on the plasma level of that substance & the glomerular filtration rate (GFR)

Transport maximum Is the maximal rate at which substances can be transported across the tubular cells into the blood per minute (This maximum limit is due to saturation of the specific carrier systems involved in the process The transport maximum for glucose = 320 mg/min This means that a maximum of 320 mg of glucose can be transported per minute from the tubular cells of both kidneys into the blood

What happens in Hyperglycemia??? As tubular load exceeds transport maximum Glucose will appear in urine (Glycosuria)

Renal glycosuria Is the presence of glucose in the urine even when the renal threshold for glucose is not exceeded It occurs due to a renal disease, which is associated with a low renal threshold for glucose

References CL Ghai MBBS MD, “A Textbook of Practical Physiology”; 7th edition