Diagnostic Use

Diagnostic Use and Interpretation

After blood collection, blood cells continue to metabolise glucose in the sample and it is estimated that glucose decreases by 0.2-0.6 mmol/L per hour in whole blood (1-3), depending on the glucose concentration, temperature, leukocyte count, etc, and this can also vary between individuals.

Fluoride is an inhibitor of glucose metabolism but it takes about 1 hour to work. Collection into a fluoride tube can prevent the decrease of glucose in whole blood beyond the first hour.

Collection in non-fluoride tube is acceptable when samples can arrive at the lab within 1 hour post-collection because the separation of plasma/serum from the blood cells by centrifugation in the lab can stop the glucose metabolism. Therefore, PST tubes are the preferred specimen for collection within ADHB as other biochemistry tests can be done on the same tube.

When interpreting glucose results, individual biological variation should be taken into account. The within-subject biological variation for glucose is 5% (4) and with 95% confidence interval, the variation would be +/- 10% within an individual. For example, a fasting glucose of 6.0 mmol/L means that it can range from 5.4 – 6.6 mmol/L within that individual.

Furthermore, there are other factors affecting the glucose levels including acute illness, stress, medications, diet patterns, physical activity, etc (5).

Stated fasting glucose ranges are provided as a guide to interpretation in clinically stable patients, and should not be used for diagnosis if the patient is acutely unwell, as glucose tolerance can be temporarily significantly impaired. Equivocal results, especially near the diagnostic cut-offs, should be followed-up when patient is clinically stable.

For random (untimed) samples, a glucose result >11 mmol/L in a stable well patient is diagnostic of diabetes if symptomatic. However, the result should be confirmed with a followup glucose and also HbA1c if they are asymptomatic.

An elevated glucose does not necessarily establish the diagnosis if the patient is acutely unwell and especially if on drugs which impair glucose tolerance such as steroids. Status may need to be confirmed once clinical status has stabilised after 3-6 months.

For diagnosis of diabetes mellitus, HbA1c has the advantage of less biological variation (1.2% in healthy subjects) (4) and is less affected by acute changes (6, 7).

References
1. Chan AY, Swaminathan R, Cockram CS. Effectiveness of sodium fluoride as a preservative of glucose in blood. Clin Chem 1989;35:315-317.
2. Sacks DB, Arnold M et al. Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus. Diabetes Care. 2011;34:e61-99.
3. Carey R, Lunt H et al. Collection tubes containing citrate stabiliser over-estimate plasma glucose, when compared to other samples undergoing immediate plasma separation. Clin Biochem. 2016;49:1406-1411.
4. Carlsen S, Petersen PH, et al. Within-subject biological variation of glucose and HbA(1c) in healthy persons and in type 1 diabetes patients. Clin Chem Lab Med. 2011;49:1501-1507.
5. Good to Know: Factors Affecting Blood Glucose. Clin Diabetes. 2018;36:202.
6. Bonora E, Tuomilehto J. The pros and cons of diagnosing diabetes with A1C.Diabetes Care. 2011;34 Suppl 2:S184-190.

7. New Zealand Society for the Study of Diabetes. Executive Summary of position statement. https://www.nzssd.org.nz/HbA1c/2.%20NZSSD%20exec%20summary%20diagnosis%20of%20diabetes%20Sept%202011%20final.pdf (accessed 15/1/20)

Reference Intervals

Fasting glucose reference ranges (patients fasted for a minimum of 8h).

Random non-fasting glucose Point of Care Testing (POCT) ranges:

Pregnancy: see “Gestational Diabetes”

Conversion Factors : mg/100 mL x 0.0555 = mmol/L
mmol/L x 18 = mg/100mL

Uncertainty of measurement:  5%

** For Whole Blood Uncertainty of measurement refer to the Blood Gases page