Diagnosing Diabetes When The Test Don't Agree

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A 48 y/o man with a family history of type 2 diabetes had screening test done about 1 year ago and had an elevation of his FBS to  108 mg/dl. His A1c was 6.1 . He as told that he had Pre - Diabetes . His other lab included a Triglyceride level of 350mg/dl , HDL 34mg/dl and LDL cholesterol  of 120 mg/dl .
 His treatment recommendation was to increase his exercise , lose weight  and recheck his lab test in 1 year.
 He followed the advice and also started Omega 3 Fatty acids , taking over 4 Grams/day.

 Lab test  one year later    - A1c 6 ( 53mm/L )    Fasting glucose 98 mg/dl   ( 5.4 mm/L ). 
                                          Cholesterol is 195 mg/dl  (5 mm/L )
                                          LDL  cholesterol is 131 mg/dl  ( 3.4mm/L )
                                          HDL is 36 mg/dl  (.9 mm/L )
                                          Triglycerides have decreased to 148mg/dl ( 1.7 mm/L )

Question . Which test , A1c or Fasting Glucose , is the most useful for diagnosing Pre-diabetes

                                                      Reference Lab.

Criteria for the diagnosis of Diabetes         Criteria for diagnosis of Pre-Diabetes
 A1c  > 6.5                                                       A1c     5.7 to 6.4
 Fasting Glucose > 126 mg/dl  ( 7 mm/l )         Fasting glucose  100mg/dl to 125 mg/dl
                                                                                                 ( 5.6 mm/l) to ( 6.9mm/l )

Comment. 


 These lab results raise some interesting questions about our diagnostic tools .

  The A1c test is generally thought to be less sensitive for the diagnosis of Diabetes than the Fasting
  Glucose. The A1c results can be effected by pregnancy , hemolytic anemias , recent blood loss and
  blood transfusions.


  The American Diabetes Association suggested approach to the diagnosis of   diabetes when A1c     and Fasting glucose results do not agree is :

'The test  whose result is above the  diagnostic cut point should be repeated , and the diagnosis
 in made on the basis of the confirmed test"


There is another interesting caveat when comparing  A1c results with Fasting Glucose  results   Both fasting glucose or after meal glucose  contributes  to the A1c test.


The A1c test is an average of all glucose results for the previous 2 to 3 months. 


For example the after meal glucose results contribute  only about  25 % to an A1c level of 10.2 but contributes 75% to an A1c result of 7.3.


This means that the closer the A1c results get to 6.5 The greater the after meal sugar is  contributing to the result. 


With the above thoughts in mind I would tend to believe the A1c result and follow the ADA recommendations and repeat the A1c  test. 

p.s.
 This information is also useful for helping you decide whether to do more after meal test  as your A1c hemoglobin approaches your A1c goal of  7


Tomorrow. What to do about the lipid test results ?


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 David Calder, MD                                     

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