Glucose Tolerance Tests

Contact the duty biochemist on 0121 3716543 to discuss whether an oral glucose tolerance test (OGTT) is required for a particular patient and/or to book an OGTT. The OGTT is performed at the Diabetes Centre Laboratory in Nuffield House on the QEHB site. In addition, a protocol can be provided for performing an OGTT on wards or in the community.

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Monitoring glycaemic control in patients with diabetes

HbA1c is routinely measured for this purpose. The assay is run daily on weekdays and requires an EDTA plasma sample (purple top). Fructosamine can be used as an alternative when HbA1c is not appropriate. Fructosamine reflects blood glucose over two weeks rather than 2 to 3 months as it reflects glycation of albumin rather than haemoglobin. Fructosamine is performed on serum (yellow or red top) and run daily.

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Fluid Analysis Guidelines

Below are some guidelines for fluid analyses which may be of clinical value. Please contact the Duty Biochemist on x16543 if more information is required:

Fluid type

Clinical Indication

Analyses available

Specimen Container

Comments

Ascitic Fluid

? cirrhotic or malignant

Albumin

Total protein

Cholesterol

Triglycerides

LDH

Plain universal

Serum albumin should be simultaneously measured for comparison.

? SBP

 

pH

 

Total protein

See comment

 

Plain universal

For rare instances pH should be collected anaerobically with heparin and then measured in a blood gas analyser using clot filter.

? pancreatic fistula

?pancreatitis

Amylase

Plain universal

Serum amylase should be measured.

? tubercular

Glucose

Fluoride oxalate (grey top)

 

Chest Drain Fluid

? chylothorax

Chylomicrons

Cholesterol

Triglycerides

Plain universal

 

 

CSF

? bacterial meningitis

Protein

 

Glucose

 

Plain universal

 

 

 

Fluoride oxalate (grey top)

 

? Subarachnoid haemorrhage

Xanthochromia

Plain universal protected from light

Do not use pod system to send sample to lab.

Serum total protein and bilirubin should be measured simultaneously.

?congenital disorder

?cerebral ischaemia

Lactate

Fluoride oxalate (grey top)

Sent to BCH Biochemistry.

?brain metastases

AFP, HCG, placental ALP

Plain Universal

Sent to Charing Cross for analysis.

Diagnosis/investigation of inborn errors of neurotransmitter metabolism

Neurotransmitters

See comment

Specific collection requirements  – contact Duty Biochemist on ext. 16543 well in advance of arranging test.

Sent to Neuroimmunology lab, London.

?narcolepsy with cataplexy

Orexin/Hypocretin

Plain Universal

Sent to Immunology, Oxford.

?neurosarcoidosis

ACE

Plain Universal

CSF total protein also required for interpretation.

Sent to Neurometabolic unit, London.

Cyst Fluid

?thyroid tissue/met

Thyroglobulin

Plain universal

Requires discussion with laboratory prior to request (contact Duty Biochemist on ext. 16543)

Drain Fluid

? contains urine

Urea

Creatinine

Plain universal

Comparison of fluid urea and creatinine with serum will identify significant contamination with urine

?biliary fistula

Post surgery

Amylase

Bilirubin

Plain universal

 

Gastric Aspirate

? reflux

?achlorhydria

pH

 

Plain universal

Occasionally gastric pH may be requested in patients suspected of intestinal reflux or achlorhydria. Normally the fasting gastric pH is about 1-2.

Pancreatic Cyst Fluid

 

? Ca pancreas

CEA

CA 19-9

Amylase

Glucose

Plain universal

 

 

Fluoride oxalate (Grey top) required

 

 

 

 

Pleural Fluid

Four types of fluids can accumulate in the pleural space:

  • Serous fluid (hydrothorax)
  • Blood (haemothorax)
  • Chyle (chylothorax)
  • Pus (pyothorax or empyema)

 ? transudate or exudates

 

A transudate fluid is produced through pressure filtration without capillary injury while exudate is "inflammatory fluid" leaking between cells.

Most common causes of pleural exudates are bacterial pneumonia and malignancy.

Most common causes of pleural transudates are left ventricular failure and cirrhosis.

Total Protein

LDH

 

Plain universal

TP <25g/L indicates transudate.

TP >35g/L indicates exudate.

 

Light’s criteria applies to pleural fluid TP between 25 and 35g/L.

A fluid is an exudate if any of the following apply:

Ratio of fluid protein to serum protein is >0.5

Ratio of fluid LDH to serum LDH is >0.6

Pleural fluid LDH is > 2/3rds the upper reference limit for plasma LDH.

 

Measure serum protein and LDH simultaneously

? infected

pH

 

See comment

This is part of British Thoracic Society’s guidelines for differentiating infective from non-infective pleural effusions, can only be measured on fresh specimen collected anaerobically using a dedicated blood gas analyzer. This analyser can be found on W513 (respiratory).

? chylothorax

Chylomicrons

Cholesterol

Triglyceride

Plain universal

 

? pancreatitis

Amylase

Plain universal

Patient's serum amylase should be measured for comparison.

? rheumatic cause

Glucose

Fluoride oxalate (grey top) tube required.

 

Nasal Fluid

? CSF

Tau protein

Plain universal

Sent to Immunology, Sheffield.

Salivary Cortisol

?Cushing’s

ONDST

Salivary Cortisol

Salivette or Plain universal

Saliva specimens should be collected using a Sarstedt cortisol salivette (these can be requested from Chromatography). Saliva collected into a plain container by passive drool is also acceptable. 

Synovial Fluid

Refer to Microbiology

N/A

 

N/A.

Urine pH

?cause of metabolic acidosis

pH

Plain universal

In patients with a metabolic acidosis and suspected renal tubular acidosis, urine pH measurement is indicated.

 

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Diagnosis of Diabetes Mellitus

Conventionally diabetes mellitus was diagnosed by high fasting or random blood glucose concentrations, or an abnormal oral glucose tolerance test (OGTT) whilst haemoglobin A1c (HbA1c) was used to monitor longer term glycaemic control in patients with known diabetes mellitus.

In 2011, the World Health Organisation (WHO 2011) recommended that HbA1c measurements should also be used to diagnose diabetes in the majority of asymptomatic individuals, and this recommendation has been agreed in the UK (NHS Diabetes 2011).

An HbA1c of 48 mmol/mol or more is consistent with diabetes. If the patient has no symptoms then a second HbA1c result must be obtained within 2 weeks, and if it remains ≥48 mmol/mol diabetes mellitus is confirmed.

HbA1c values of 42 to 47 mmol/mol suggest a high risk of future diabetes. Such individuals should be offered structured lifestyle education and support to delay/prevent development of diabetes, and have an annual HbA1c test.

HbA1c must be measured in an accredited laboratory undertaking recommended quality assurance procedures. Near patient testing is not appropriate when HbA1c is used for the diagnosis of diabetes.

HbA1c is now the preferred method to diagnose diabetes, except in the following situations where this test would be unreliable, and in whom the traditional methods of diagnosis with blood glucose concentrations remain the method of choice:

  • Haemoglobinopathies
  • Increased red cell turnover
  • Anaemia (haemoglobin < 80 g/L)
  • ?Type 1 diabetes or acute onset of symptoms of diabetes
  • ?Gestational diabetes
  • Children and adolescents
  • Patients taking steroids and antipsychotic or other medications that cause a rapid rise in blood glucose

Despite this new approach, if an individual has abnormally high random or fasting blood glucose levels or abnormal OGTT, which would be consistent with diabetes on the traditional criteria, then that patient should be considered to have diabetes irrespective of their HbA1c value. Without symptoms of diabetes two abnormal tests of the same type (two high fasting/random blood glucoses or a diabetic OGTT) are required to confirm diabetes mellitus.

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