Clinical biochemistry notes
(Aotea News, March 2009)
Inappropriate use of the oral glucose tolerance test
The OGTT is not always required to diagnose diabetes and definitely should not be the first line test in screening.
Fasting plasma glucose is the recommended screening test and is usually carried out in the context of a full cardiovascular risk assessment.
Recently, there have been many inappropriate requests for the OGTT after a screening test with fasting plasma glucose result showing very high levels, consistent with diabetes.
In some of these cases the patients have had obvious signs and symptoms of hyperglycaemia and then the single high fasting level, e.g. 11-16mmol/L, is sufficient to make the diagnosis.
In fact, any glucose ≥ 11.1 mmol/L, whether fasting or random, is diagnostic if there are signs and symptoms of hyperglycaemia as well. In other cases, when the initial fasting level reaches the threshold for diabetes in a patient without clinical signs and symptoms, the correct follow up test is a repeat fasting plasma glucose.
A second level over the threshold, ≥ 7.0mmmol/L, confirms diabetes.
The OGTT should be reserved for any patient who has either impaired fasting glucose, 6.1-6.9mmol/L or who is at high risk of diabetes and has fasting glucose 5.6 – 6.0 mmol/L.
These procedures for screening and diagnosis of diabetes have not changed from those recommended in 1999 and 2003 and recently have been restated as current practice by the New Society for the Study of Diabetes. These criteria are also those recommended by bpac, NZGG and the Royal College of Pathologists of Australasia.
There have been a number of requests for the lactose tolerance test. This is not recommended for testing for lactose intolerance and the test is no longer available in the Wellington region.
The test has very poor sensitivity and specificity for lactose intolerance and, strictly, requires a follow up glucose tolerance test to be performed in the case of a positive result.
Withdrawing lactose and observation of symptoms, with rechallenge if necessary, is a more powerful predictor than the tolerance test. This approach is endorsed by bpac. If suspicion remains then specialist referral may be indicated.
hs-CRP and CRP
The test called high sensitivity C Reactive Protein (hs-CRP) measures exactly the same substance as the routine test for CRP but is calibrated to measure at much lower levels, in the range of 0-3mg/L.
Epidemiological evidence shows an association of cardiovascular risk with low levels of CRP, risk increasing as CRP increases across the population reference interval.
However, the data has not been sufficiently strong to recommend using hs-CRP in routine cardiovascular risk assessment.
After the recent publication of the JUPITER trial hs-CRP has been in the news, with some calling for more routine use of the test to help decide eligibility for statins (see NZ Doctor 3rd December 2008 for discussion).
After consideration of this new data and existing data, the New Zealand Guidelines Group has recommended that hs-CRP is not indicated in routine risk assessment (current at January 2009).
In fact, the test for hs-CRP is not widely available across New Zealand. There is concern that some practitioners may be using the routine test for CRP as a surrogate for hs-CRP. The routine test is not suitable for that purpose as it is inaccurate in the low range. Thus CRP will now be reported only down to a level of 3mg/L.
This is sufficient for use in inflammatory and infectious conditions but will preclude any attempted use in cardiovascular risk stratification.
If there are special clinical circumstances which suggest that hsCRP is indicated, these should be noted very clearly on the request form and it may be possible for the testing to be arranged.