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Therapeutic Drug Monitoring (TDM)

A TDM service is provided in the department of Clinical Biochemistry for some of the drugs requiring regular monitoring. Please check the time at which a sample should be collected as failure to collect the blood at the appropriate time will make it impossible to compare the measured concentration with an accepted therapeutic range. Assays are of little use under these conditions.

The monitoring of valproic acid is not recommended because of the poor correlation between plasma levels and therapeutic effect. The duty biochemist must be contacted to discuss its measurement for any other purpose.

Any request for urgent therapeutic drug analysis that is not provided by Clinical Biochemistry must be discussed with the Duty Biochemist, or when out-of-hours with the duty Biomedical Scientist who may ask you to discuss this with the Duty Consultant. In some circumstances additional discussion with a pharmacist is required. Some specimens requiring analysis out-of-hours may need to be sent to the Toxicology unit at City Hospital and if you organise this without consulting the Pharmacy department you will incur charges to the trust that will not be the responsibility of Clinical Biochemistry.

Overdoses/Drug Screens

Samples for a 'Drug Screen' are analysed at the Regional Toxicology Laboratory (RTL) at City Hospital. A request for a "drug screen" or ‘unknown drug’ requires:-

  • At least 10 mL of urine in plain tube (NOT containing boric acid).  A green top (Lithium heparin anticoagulant) tube filled with blood.  Detailed patient information.
  • Clinical condition e.g. coma grade, fitting etc.
  • Current known prescribed drugs.
  • Overdose drug(s) if known.
  • Urine specimen type e.g. voided or catheter.

Blood samples taken for a ‘Drug Screen’ are of little use unless there is prior knowledge of the agent ingested. Discuss whether a blood sample is of use with the duty biochemist or duty consultant (via the out-of-hours Biomedical Scientist). Paracetamol, salicylate and lithium measurements are available on a 24 hr basis. All requests for other ingested drugs must be discussed with the Poisons Unit and Regional Toxicology Laboratory and samples sent only by prior arrangement. Any samples requiring urgent analysis at the Regional Toxicology Laboratory will require direct dispatch to that laboratory and must not be sent to the Clinical Biochemistry department. Charges realised by these analysis will be forwarded to the relevant division.

Antibiotics

Amikacin, gentamycin, tobramycin and vancomycin are measured in Clinical Biochemistry but clinical advice is given from Microbiology. For patients with liver or renal impairment, advice on antibiotic dosage is available from Clinical Microbiology.

Assays for other antimicrobials including flucytosine, teicoplanin and streptomycin are available from Microbiology after prior consultation with the Clinical Microbiologist.

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Thyroid Function Tests

TSH and free thyroxine (fT4) are provided as first-line tests. Since many drugs and treatments affect thyroid function tests, details of all drugs or other treatment must be provided in order that further tests can be initiated by the laboratory as appropriate. Please indicate on request form if patient is on thyroid hormone replacement.

Free T3 is analysed only according to an agreed protocol and full clinical details must be given on the request form.

Thyroid hormone measurements can be misleading in patients with acute and non-thyroid illness. Thyroid status should only be assessed after recovery from acute non-thyroidal illness. 'Screening' of patients in hospital for thyroid illness is not recommended.

Please see the UK Guidelines for the Use of Thyroid Function Tests at: http://www.british-thyroid-association.org/current-bta-guidelines-

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Management of the Menopause

The menopausal transition is best diagnosed on clinical grounds. Endocrine investigation may be helpful where the pattern of age, menstrual history and features of oestrogen deficiency are unusual.

Please indicate the woman’s date of birth, recent menstrual pattern and date of last menstrual period/day of cycle on which the blood sample was collected. A rise in follicle stimulating hormone (FSH) is the earliest sign of the approaching menopause. Measurement of serum FSH is the recommended first investigation if biochemical confirmation is necessary. The measurement of luteinising hormone (LH), oestradiol or progesterone is not appropriate. A serum FSH in the reference range for the follicular phase does not exclude the perimenopause.

Hormone Replacement Therapy

HRT when prescribed (orally or transdermally) for the relief of menopausal symptoms does not require endocrine monitoring. Where there is unexpected failure of treatment, for example due to non-compliance or malabsorption, investigation may be useful. Different formulations of HRT may or may not be detected by oestradiol assays. Please indicate on the request form the HRT preparation prescribed.

The main indication for measuring oestradiol in women on HRT is in those receiving implants containing oestradiol. Early replacement of the implant may result in accumulation of oestradiol. Monitoring of serum oestradiol before the implant is replaced has been recommended to avoid supraphysiological concentrations. Sometimes testosterone implants are used in HRT. Measurement of testosterone in an analogous fashion to oestradiol may help to assess whether a further implant may be necessary.

 

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Coronary Heart Disease Risk Score

Standard 4 of the National Service Framework for Coronary Heart Disease states that:

“General Practitioners and Primary Health Care teams should identify all people at significant risk of cardiovascular disease (CVD) but who have not yet developed symptoms and offer them appropriate advice and treatment to reduce their risk”.

Numerous risk calculators are available to calculate risk for coronary disease and these are all based on the Framingham model. The JBS III or the QRISK 2016 calculators are used for the calculation of cardiovascular disease risk. The South Birmingham PCT requires general practice to screen for CVD and any request for ‘CVD risk’ will generate a total cholesterol, HDL-cholesterol, creatinine and HbA1c (a yellow top and purple top bottle must be supplied). There are freely available calculators available to calculate a ‘CHD risk’ (Coronary heart disease risk) score (e.g. http://www.qintervention.org/)

Please note HDL-cholesterol is only measured when the CHD risk score is requested. We do not provide HDL-cholesterol otherwise on any request for a lipid profile.

Often we are asked to provide LDL-cholesterol calculations. For your convenience the calculation of LDL-cholesterol is provided below but you should recognise that this is only strictly valid where patients attend fasting for at least 12 hours in order to suppress triglyceride concentrations. Triglyceride concentrations >4.5 mmol/L negate the use of the calculation;

LDL cholesterol = Total cholesterol – HDL-cholesterol – (Triglyceride/2.19)

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