Analysis strategies

A request for thyroid function tests (TFTs) will generate a thyroid stimulating hormone (TSH) in the first instance. If this is within the reference range and there is no indication from the request form that thyroid hormone measurement would be helpful - such as in cases of hypopituitarism - no further tests will be done.

A free thyroxine (fT4) will automatically be measured on all samples with a TSH outside the reference range. If this does not yield or substantiate the clinical diagnosis, a free triiodothyronine (fT3) measurement will be made.

Please always indicate if the patient is receiving either thyroid replacement therapy. Please specify T4 (or rarely T3 tablets), or anti-thyroid drugs. This will help the laboratory to provide a more streamlined and effective thyroid service.

Thyroid function is disturbed in seriously ill patients. Investigation of suspected thyroid disease should, therefore, be delayed until any acute episode has resolved. For this reason, TFTs are not recommended on emergency admissions unless thyroid disease is strongly suspected.

The ideal tumour marker has not yet been identified. 

Increased concentrations can be caused by certain non-malignant conditions thus care must be taken with interpretation. Tumour markers should not be used for unselected population screening. Their major role is in the assessment and follow-up of patients with cancer. The requesting clinician should, therefore, always specify exactly which tumour markers are required and not simply ask for ”Tumour Markers”. The following table lists some of the better established applications of the tumour markers which are available in-house.

The following tumour markers are available in-house.

*patients should be registered with a Trophoblastic Tumour Treatment Centre, in Sheffield.

Tumour markers not offered as in-house investigations can be provided by other centres. Please contact the laboratory for further information.

b-Human chorionic gonadotropin (b-HCG) becomes detectable in the maternal circulation about seven days after fertilisation - about the time of implantation.

It is measurable about a week before the first missed menstrual period. Thereafter the concentration rises rapidly, doubling every two to three days until it peaks at about ten weeks of gestation.

Serum b-HCG should not be used as a routine pregnancy test. It has, however, an important role in the diagnosis and management of suspected ectopic pregnancies, threatened abortions and early pregnancies in which the viability is in doubt.

The quoted therapeutic range and measured serum concentration of a drug provides guidance only.

There is wide inter-individual variation and the patient, not the biochemical result, should always be treated.

Requests for TDM should include details of:

• drug therapy
• dose
• dosage frequency
• date and time of last dose
• route of administration
• date and time of sample

The reason for the TDM request should always be stated. For example, toxicity, compliance or change of dose.

For drugs given orally it takes approximately 5 half lives (t1/2) to reach steady state.

After starting treatment or changing the dose, requests for TDM should not be made until 5 half lives have elapsed and steady state has been achieved.

Trough values are generally to be preferred but peak concentrations may be useful to confirm toxicity.

A limited toxicology service is provided in-house. The serum drug analyses detailed in the out-of-hours section are available at all times. A screening service for urine drugs of abuse is provided for the Substance Misuse Team (see below) and may be used by other clinicians but this is not available as an emergency service. Analysis is performed twice weekly, usually on a Tuesday and Friday. When necessary, other drug analyses are available from referral centres. It is essential, however, that some indication is given of the suspected drug or group of drugs involved. Please discuss such requests with the laboratory.

In addition to those investigations required for immediate patient management the following samples should be obtained from overdose patients:

• 4.9 mL clotted blood (brown top tube)
• 20 mL first urine obtained
• 20 mL gastric washings (if available)

These will be stored by the laboratory for 10 days and can be analysed should the need arise.

The laboratory offers a limited, urine drugs of abuse screen which covers the following drugs:

• amphetamines
• benzodiazepines
• cannabinoids
• cocaine
• methadone
• opiates

Urine drugs of abuse screening is done by immunoassay twice weekly, usually on a Tuesday and Friday. A confirmatory chromatography service is not routinely provided in-house but, if specifically requested once the results are available, samples that are positive on the preliminary screen can be sent away for further analysis.

The service is not available out-of-hours or on an urgent or emergency basis. Any such requests must be discussed with the Consultant Biochemist (extension 5749 or 2772)

Please note - the service is provided for clinical purposes only.

Glycated haemoglobin was identified in 1958 and its use in monitoring glycaemic control in diabetics was first proposed in 1976. Further characterisation and the introduction of an international standard, as well as improvements in analytical methods, has allowed the test to be certified by the World Health Organisation (WHO) and the International Federation of Clinical Chemistry (IFCC) as traceable to this and has, since the early 2000s, been used in diagnosing DM and impaired glucose tolerance. However, in conditions that affect lifespan of red blood cells, the test may give falsely elevated or falsely low results, not necessarily reflective of glycaemic control.

NICE have recommended the following limitations on use of HbA1c

HbA1c should not be used to diagnose diabetes mellitus in the following groups:

• children and young people less than 18 years of age
• pregnant women or women who are 2 months postpartum
• people with symptoms of diabetes for less than 2 months
• people at high diabetes risk who are acutely ill
• people taking medication that may cause hyperglycaemia (for example long-term corticosteroid treatment)
• people with acute pancreatic damage, including pancreatic surgery
• people with end-stage renal disease (ESRD). See the CKS topic on chronic kidney disease for more information
• people with HIV infection - see the CKS topic on HIV infection and AIDS for more information

HbA1c should be interpreted with caution in people with abnormal red blood cell turnover or abnormal haemoglobin type:

• abnormal haemoglobin, such as haemoglobinopathy
• severe anaemia (any cause) - see the CKS topics on Anaemia – iron deficiency and Anaemia – B12 and folate deficiency for more information
• altered red cell lifespan (for example post-splenectomy)
• a recent blood transfusion

[WHO, 2011; Kilpatrick and Atkin, 2014; Davies, 2018; Milne, 2020; NICE, 2020a]

If you would like further clarification, please contact the laboratory.

There was a recent case report in the BMJ from Leeds about a case of lead poisoning in a child that sadly led to their death. The case highlighted that lead poisoning can be a possible cause of iron-deficiency anaemia in children and should be considered as a possible cause of unexplained, with/without a presentation of pica.

We have included a new pop-up on ICE when ferritin is requested in children.