Home / Resources / Clinical Gems / Joslin’s Diabetes Deskbook, Updated 2nd Ed, Excerpt #6: The Approach to Diagnosis and Treatment

Joslin’s Diabetes Deskbook, Updated 2nd Ed, Excerpt #6: The Approach to Diagnosis and Treatment

May 20, 2012

By Richard S. Beaser, M.D., and Richard Jackson, MD



If an oral glucose tolerance test (OGTT) is done and a diagnosis of diabetes is confirmed, do you need to perform another test? Do you know the procedures to perform an OGTT?….




Diagnosing Diabetes

The first challenge in treating diabetes is making the diagnosis. This may sound like an obvious statement until you realize that about forty percent of all people who actually have diabetes have not yet been diagnosed. Most of these, of course, have type 2 diabetes, which can develop insidiously and have such subtle onset of symptoms that they are hardly noticed or are blamed on something else. Yet, by the time of diagnosis, the disease has had a head start in causing the chronic complications of diabetes.

Follow-up studies of the first two large clinical trials showing the positive effects of lower A1C values on reducing complications in both type 1 (DCCT) and type 2 (UKPDS) diabetes show strikingly similar results. The benefits of attaining good glycemic control early in the natural history of these conditions last for years, and if early glycemic control wasn’t optimal, it may take decades before a person can ‘catch up’ in lowering his or her risk for developing diabetes complications. These findings, discussed in more detail later, serve as a driving force behind the need to screen at-risk people to identify undiagnosed diabetes. Because of a number of practical difficulties, including the populations targeted, and follow-up, screening programs in the community have been unsuccessful. At this time, screening is best done in the healthcare setting, at a provider’s office, in a clinic, or at a hospital. As there is a need to make early diagnoses, it is important to screen people at-risk for undiagnosed diabetes everyone to three years. And the number of your patients who are at risk is high:

  • Anyone over 45, or
  • Anyone overweight (BMI >25), and has one of the following characteristics:
    • is non-Causasian
    • has pre-diabetes (impaired fasting glucose, or impaired glucose tolerance, see definitions in table 1)
    • has had gestational diabetes or an infant over 9 pounds
    • has other components of the metabolic syndrome
      • Blood pressure _ 140/90
      • HDL cholesterol < 35
      • Triglycerides > 250
    • has conditions associated with insulin resistance
      • acanthosis nigricans
      • polycystic ovary disease
Diagnostic criteria are discussed in detail below. The screening test can be either a fasting glucose, an oral glucose tolerance test, or an A1C.
Diagnostic Criteria 

The current diagnostic criteria for diabetes are summarized in Table 2-1. The current recommended method for diagnosing diabetes is to use the venous fasting glucose level. Based on this measurement, diabetes is defined as a glucose level 126 mg/dl, confirmed on repeat determination. A “casual” (new terminology replacing “random”) glucose level >200 mg/dl in the presence of classic symptoms would also make the diagnosis. The traditional oral glucose tolerance test (OGTT) is also still valid; however, as it is cumbersome to perform, it is not needed if a patient meets other criteria. Usually, an OGTT would be indicated if diabetes is strongly suspected and other test results are equivocal. If either the fasting glucose or the 2-hour OGTT result is used for the diagnosis, a confirmatory test (fasting glucose, or OGTT, whichever was used initially) needs to done on another day to secure the diagnosis. If the diagnosis is made on the basis of a high blood glucose and classic symptoms, a confirmatory test is not needed. Guidelines for screening pregnant women for gestational diabetes are different, and are covered in Chapter 20. These guidelines eliminate the need to evaluate women at low risk for gestational diabetes.


Fingerstick glucose measurement should not be used to diagnose diabetes. Also, urine testing should not be used to either diagnose or screen for diabetes. Blood glucose levels could be high enough to represent diabetes, but glycosuria may not be present due to a high renal threshold. Conversely, the presence of glucose in the urine without elevated blood glucose is not diagnostic for diabetes. Renal glycosuria, with the presence of urinary glucose but normal blood glucose levels, can be common in children. If glucose in the urine is detected, blood testing should be done.


At the time of this publication, a recommendation has been made by an expert international committee to use the A1C test as the primary approach for diagnosis and screening of diabetes. This makes a lot of sense since the A1C doesn’t need to be performed when the patient is fasting, and is the test which is used to follow metabolic control once the diagnosis of diabetes has been made. It wasn’t used in the past because of problems with consistency — different assays measuring different forms of glycosylated hemoglobin produced a variety of results. The A1C test is now well standardized, and actually has less variability than glucose measurements. A1C is also more stable after collection than glucose. The proposed cutoff for diagnosis of diabetes is an A1C > 6.5%.


Copyright © 2010 by Joslin Diabetes Center. All rights reserved. Reprinted with permission. Neither this book nor any part thereof may be reproduced or distributed in any form or by any means without permission in writing from Joslin. Note: Joslin does not endorse products or services.

For Excerpt #1 from the Joslin Diabetes Deskbook, 2nd Ed, in this series, just click here.

For Excerpt #2 from the Joslin Diabetes Deskbook, 2nd Ed, in this series, just click here.

For Excerpt #3 from the Joslin Diabetes Deskbook, 2nd Ed, in this series, just click here.

For Excerpt #4 from the Joslin Diabetes Deskbook, 2nd Ed, in this series, just click here. 

For Excerpt #5 from the Joslin Diabetes Deskbook, 2nd Ed, in this series, just click here. 

You can purchase the updated 2nd Edition of JOSLIN’S DIABETES DESKBOOK at:


Please Note: Reasonable measures have been taken to ensure the accuracy of the information presented herein. However, drug information may change at any time and without notice and all readers are cautioned to consult the manufacturer’s packaging inserts before prescribing medication. Joslin Diabetes Center cannot ensure the safety or efficacy of any product described in this book.

Professionals must use their own professional medical judgment, training and experience and should not rely solely on the information provided in this book to make recommendations to patients with regard to nutrition or exercise or to prescribe medications.

This book is not intended to encourage the treatment of illness, disease or any other medical problem by the layperson. Any application of the recommendations set forth in the following pages is at the reader’s discretion and sole risk. Laypersons are strongly advised to consult a physician or other healthcare professional before altering or undertaking any exercise or nutritional program or before taking any medication referred to in this book.