Dynamic Function Tests

The investigation of certain conditions, in particular endocrine disorders, requires stimulation or suppression tests. Please contact the laboratory for further advice and information. When using electronic reqesting for dynamic function tests guidance is provided.
Click here to view the DFT procedures available (Select; Procedures/Medicine and Specialist Medicine/Diabetology and Endocrinology).

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Thyroid Strategy

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 eg. 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 and, if this does not yield or substantiate the clinical diagnosis, a free tri-iodothyronine (fT3) measurement will be made.

Please always indicate if the patient is receiving either thyroid replacement therapy, specifying whether T4 or rarely T3 tablets, or anti-thyroid drugs as 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.

Tumour Markers

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:

  • Tumour Marker Malignancy
    AFP Hepatoblastoma
    Hepatocellular carcinoma
    Gonadal and extra-gonadal germ cell tumours (with b-HCG)
    b-HCG Choriocarcinoma*
    Hydatidiform mole*
    Gonadal and extra-gonadal germ cell tumours (with AFP)
    CA 125 Ovarian malignancy
    CA 199 Pancreatic cancer
    CEA Colorectal cancer
    PSA Prostate cancer

 

 

 

 

 

 

 

 

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

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

β-HCG In Pregnancy

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

Therapeutic Drug Monitoring (TDM)

The quoted therapeutic range and measured serum concentration of a drug provide 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 eg. ?toxicity, ?compliance, 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.

The following table summarises the analyses available in the laboratory and provides guidance for sampling times for oral administration.

Drug Half life Time to steady state Sampling time for trough Sampling time
for peak
Carbamazepine 25 – 45 h
(single)
8 – 24 h (chronic)
2 – 6 days Pre-dose 3 – 4 h post-dose
Digoxin 36 – 48 h 5 – 7 days Pre-dose or
6 h post-dose
 
Lithium 10 – 35 h 3 – 7 days 12 h post-dose  
Phenobarbitone 100 h 8 – 15 days Not vital
(Long t1/2)
 
Phenytoin 9 – 22 h (single)
20 – 40 h (chronic)
>8 days Pre-dose 3 – 4 h post-dose
Theophylline 6 h
(range 3 – 13h)
>2 days Pre-dose

Formulation dependent:
Rapid release – 2 h
Slow release – 4 h

Valproate* 7 – 16 h 2 – 3 days Pre-dose 3 – 4 h post-dose

* TDM not useful – only to establish whether compliant.

Toxicology

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.

Drugs of Abuse

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/emergency basis. Any such requests must be discussed with the Consultant Biochemist (ext 5749 or 2772)

NB. The service is provided for clinical purposes only.