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Common Factors Affecting Analysis

It is not practical to list all factors that may affect all analytes however but a guide to some of the more common factors are as follows:

  • Haemolysis, icteric and lipaemia interfere with certain analytes and as a consequence some analytes may not be reported. However, a comment will be included on the report to indicate why. Common analytes affected include: sodium, potassium, bilirubin, magnesium, phosphate, LDH, AST, ALT and cTnT hs.
  • Serum samples should be processed (centrifuged and serum separated from the cells) within 12 h of collection. Any undue delay, particularly more than 6 h, can influence the potassium and enzyme results.
  • Extreme temperatures (cold and hot) can cause abnormal levels of some analytes especially potassium.
  • Sodium is affected by abnormal levels of protein and lipids. A direct ISE measurement will be performed and this report (plus appropriate comment) will be issued.
  • A high platelet and white blood count can cause a falsely elevated potassium (a condition known as pseudohyperkalaemia). It is suggested that in these suspected cases blood is collected into both a lithium heparin tube and a yellow top serum tube and sent to the Biochemistry department as soon as possible for potassium analysis to confirm. There is a difference of approximately 0.3 mmol/L in plasma compared with serum potassium. Larger differences are consistent with blood abnormalities affecting the results.
  • It is important that blood is collected in the correct tubes and in the correct order to reduce the risk of anti-coagulant interference e.g. EDTA interference with calcium assays.
  • CSF samples for xanthochromia should be protected from light and should not transported to the laboratory using the SDS. Lumbar puncture for xanthochromia should not be performed until 12 hours post onset of symptoms in order to minimise false negative results.
  • HbA1c will not be reported on patients with known haemoglobinopathies. It is recommended that fructosamine is measured on these patients.

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)

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.

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.

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.

 

Point-of-Care Testing (POCT or Near Patient Testing, NPT)

The use of all point-of-care devices from simple dip-stick tests to blood gas analysis machines is governed by a comprehensive Trust policy and procedure. Please ensure that you are aware of the requirements of this policy before you embark on point-of-care testing. The POCT services managed by the team are rapidly expanding and include:

Blood Glucose Meters

Ward based glucose meters that measure blood glucose using a 'dry-chemistry' stick are used throughout the Trust. These meters must only be used by authorised staff that have received training via the scheme organised in conjunction with the meter manufacturer.

The working range of all glucose meters is limited and for accurate determination at the extremes of the range (2.5 - 20.0 mmol/L for the UHB meters), blood should be taken into a grey-top vacuette tube and sent to the laboratory. If the result is unexpected, send a sample to the laboratory or a sample can be processed on one of the blood gas analysers located throughout the Trust.

In hypoglycaemia, values below the working range must be confirmed by a laboratory glucose measurement. Consult the biochemist/consultant on duty prior to taking the blood so that appropriate samples can be collected for the investigation of insulinoma, should this be warranted.

Blood Gas Analysers (QEHB, Heritage Building, ROH, Moseley Hall)

The department has rationalised blood gas instrumentation in the Trust and identical instruments are sited throughout the Trust. All analysers measure pH, pCO2, pO2, sodium, potassium and ionised calcium. They provide full co-oximetry and derive values for base excess and bicarbonate. Glucose and lactate measurement is available on both sites, but not on all analysers.

The blood gas analysers are located throughout QEHB, Heritage Building and the POCT team also manage analysers at the Royal Orthopaedic Hospital and Moseley Hall Hospital.

Ward based operators and doctors are trained by staff of the POCT team. The laboratories do not have blood-gas analysers in the department and there is no in-house lactate measurement.

INR Measurement

Throughout the Trust the POCT team manage the INR devices used for rapid testing. These meters have been installed in areas where a clinical need has been identified and approved. Operators are trained by the members of the POCT team.

Ketone Measurement

To comply with the DKA guidelines and to support the diagnosis and management of patients with DKA the POCT team manage the ketone meter service within the Trust. Meters have been installed in areas where a clinical need has been identified and approved. Training and full service report is provided by the POCT team.

Other POCT Services

As well as these tests the POCT team are currently managing or are reviewing the following services:

DDimer testing, urine analysis including hCG testing (pregnancy testing) and dipsticks for screening, Haemostasis testing (ACT, APTT, ROTEM, TEG), Haematology clinics, Biochemistry one stop clinics and sexual Health testing.

If any clinical areas require advice or guidance on the installation or use of POCT systems please contact us to discuss further.

The POCT team can be contacted by: Telephone – extension 15976
Bleep - 1189
Email - This email address is being protected from spambots. You need JavaScript enabled to view it.

Porphyria Screens

The laboratory performs a urine porphyrin screen in house whilst blood samples (required for latent porphyria or cutaneous porphyria) and all samples from known porphyria patients are referred to a specialist laboratory. The sample required and interpretation depends upon the clinical scenario;

  • ? acute porphyria in a symptomatic patient: Send urine to lab for a porphyrin screen (PBG and total urine porphyrin). A negative PBG screen in a symptomatic patient excludes an acute porphyria as the cause of the symptoms.
  • ? acute porphyria in an asymptomatic patient (or >3 days post-symptoms): Send urine to lab for a porphyrin screen (PBG and total urine porphyrin) and an EDTA blood to refer to a specialist laboratory to rule out latent/resolving porphyria.
  • ? bullous porphyria (skin fragility): Send urine to lab for a porphyrin screen (PBG and total urine porphyrin). A negative total urine porphyrin excludes porphyria as a cause of skin fragility.
  • ? acute photosensitivity (EPP): Send EDTA blood to lab for referral to a specialist laboratory.
  • Asymptomatic patient with a family history of porphyria: Send urine, blood & faecal samples to QEHB laboratory for referral to a specialist laboratory.

All samples for porphyria investigations must be protected from the light otherwise there are a risk of a false negative result. Clinical details MUST be provided with a request for investigation of porphyria. This is to enable appropriate investigation and reporting by both QEHB lab and the specialist laboratories.

In the event of a positive or equivocal urine porphyrin screen, additional samples (EDTA blood/faeces) may be requested for referral to a specialist laboratory.

Prolactin and Macroprolactin

The laboratory will perform a screening test for immunoglobulin bound prolactin (macroprolactin) on samples with a prolactin greater than 600 mIU/L. Please contact the laboratory for further information.

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.

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-

Troponin

1.1.       High Sensitive Troponin T has changed to High Senstive Troponin I

Interpretation is as follows:

99th Centile Males < 34 ng/L and Females < 16 ng/L

  • “Result is > 99th In cases of chest pain this could be indicative of an ACS.  Requires clinical assessment by cardiologist or cardiology nurse practitioner”
  • “Result indicates a detectable troponin level but below 99th  No evidence of an ACS but a second sample 1-3 hours apart is required to rule out myocardial infarction.
  • Result indicates no detectable troponin (<5 ng/L).  If the chest pain was > 6 hours ago this result makes an ACS unlikely and in the absence of high clinical suspicion no further troponin test is required.  
  • Troponin results cannot be used in isolation to diagnose or rule out an ACS.  Please use the HEART score and refer to cardiologist or cardiology nurse practitioner if advice is required.

 

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