Diagnosis of Diabetes

Key Points

• There are a number of tests that can be used to diagnose diabetes
  • Different tests are used depending on circumstances
  • The two key tests are Hba1c and OGTT

• Complications have their own diagnostic investigations – see separate

Urine

• • The Urine Dipstick can check for three things
    • Glucose
      • Poorly sensitive and specific.
      • Glycosuria only occurs if BGL >10mmol/L
      • Only really used for screening
    • Albumin
      • As with glucose – Stick only detects very high albuminuria at >300mg/l
      • Ketones can be identified on urine if suspecting DKA in T1 diabeticcs
      • Also excludes DDx
    • Generally, the dipstick is not very sensitive for diagnosis

Blood tests

• • 2 main blood tests are used for diagnosis:
    • HbA1c
    • Oral glucose tolerance test (OGTT)
    • There are also two other options that are now rarely used
      1. Fasting plasma glucose (i.e. no food for ≥8 hours) (>7.0)
      2. Random plasma glucose (>11.1)
    • For Hba1c and plasma glucose, we usually require  2 samples but 1 may be enough in symptomatic patients

HbA1c (Glycated haemoglobin)

• This test measures the amount of glycosylation of haemoglobin
  • Lasts over the lifetime of the erythrocyte
  • Gives a long term indication over ~3 months, and is therefore more reliable for monitoring and diagnosis.
• This is the gold standard for diagnosis – preferred over OGTT/fasting glucose unless there is a contraindication  – see below
• Threshold = > 6.5% or >48mmol/L depending on what assay is used (DCCT vs IFCC)
  • Measured twice for diagnosis
  • Currently, IFCC methods are preferred

• Beware and avoid in certain scenarios where the constitution of haemoglobin is altered:
  • RBC abnormalities – Haemoglobinopathies, anaemia, blood transfusions, and CKD
  • Children
  • Pregnant women 
  • Acutely ill patients

Venous plasma glucose

This is typically measured using a fingerprick POC test (although lab tests can be used

• Thresholds:
  • Random glucose > 11mmol/l
  • Fasting glucose > 7.0mmol/l
    • (To convert glucose from mmol/L to mg/dL multiply by 18)

• Rarely used for diagnosis in asymptomatic patients
  • Typically used for patients for self-monitoring
  • If used for diagnosis
    • Would measure twice if asymptomatic
    • If symptomatic a single value >11 is enough

Oral glucose tolerance test (OGT)

• It is very helpful in the diagnosis of Prediabetic States as well as diabetes
• This is essentially a variant of plasma glucose

Process of OGT:

• A fasting measurement is taken (as above)
  • This should be <6mmol/l in a normal patient
• Patient is then given a 75ml solution of glucose (e.g. lucozade) and the BGL is measured again in 2 hours
  • This second reading should be <7.8mmol/L in a normal person

• If either/both are high, the patient has IGT/IFG/Diabetes (See pre-diabetic conditions)

The OGT is mainly only used for diagnosis in two situations:

1. Preferred over hba1c in certain situations when the Hba1c is contraindicated
2. Used to confirm diagnosis if the fasting glucose or Hba1c result is uncertain
   • E.g. a patient is found to have a fasting plasma glucose of 6.1-7mmol/L or random glucose of 8-10mmol/L.
     • Anything over either or both of these values confirms diagnosis of diabetes

Other tests

• These are mainly needed to distinguish Type 1 vs Type 2 diabetes
  • This is important in determining the use of insulin in management

Autoantibodies

• Anti-GAD and Anti-Islet Autoantibodies are used to differentiate T1 from T2 
  • Anti-GAD is mainly used
  • Anti-Islet has poor sensitivity/specificity
• Often used where the diagnosis is unclear
  • E.g. patients >30 who present with features of Type 1 DM

C-Peptide test

• C-peptide is a byproduct of insulin synthesis and so is a useful marker of how much insulin is endogenously produced
• Therefore it is absent in T1DM (opposite to the autoantibodies)
• Indications:
  • Again distinguishes insulin resistance vs deficiency when it is clinically unclear (like antibodies above)
    • High levels = resistance
    • Absence = deficiency
  • Also useful in distinguishing endogenous vs exogenous insulin – e.g. in determining whether hypoglycaemia is due to sulphonylureas vs insulin
    • In exogenous insulin, C-peptide remains low