Patient Sample Collection and Use of the Laboratory



Patient Sample Collection and Use of the Laboratory





Most clinical laboratories take stringent precautions to control the analytical accuracy and precision of results. Indeed, many laboratories undergo carefully regulated accreditation and inspection procedures to ensure that they are fit for practice. Generally, laboratory users need only limited knowledge of the technical details of the laboratory tests. However, they should understand that the appropriate collection of patient specimens can affect results (Table 29.1), and they should therefore work with the laboratory in its attempt to produce answers rapidly and accurately, with appropriate interpretation when required and identifiable with the relevant patient. To this end, one should understand the importance of:



  • accurately completed request forms,


  • the collection of specimens by the correct technique at the appropriate time,








    Table 29.1 Some extra-laboratory factors leading to erroneous results


















































    Cause of error

    Some possible consequences


    Patient not fasting

    High plasma triglyceride and glucose


    Keeping blood overnight before sending it to the laboratory or refrigerating blood sample

    High plasma potassium, phosphate, LDH, AST


    Haemolysis of blood

    As above, lower plasma ALP


    Prolonged venous stasis during venesection

    High plasma protein, total calcium and cholesterol


    Taking blood from an arm with an infusion running into it

    Dilution of blood constituents such as electrolytes and glucose


    Putting blood into wrong vial or tipping it from one vial into another

    e.g. EDTA or oxalate cause low plasma calcium or ALP


    Blood for glucose not put into fluoride

    Low blood or plasma glucose


    Delay in analysing blood gases

    Low bicarbonate concentration


    Failure to keep sample cool or delay separating and freezing plasma

    Low PTH, ACTH, insulin


    Incorrect anticoagulant

    e.g. gut peptide hormones falsely low if no protease inhibitor used


    Palpation of prostate by rectal examination, passage of catheter, enema in last few days

    High tartrate-labile acid phospatase and PSA


    Inaccurately timed urine collection

    Poorly timed 24-h urinary excretion values


    Abnormal renal clearance values


    Urine collections without preservative

    Falsely low result, e.g. urea or calcium


    Loss of stools during faecal fat collection

    Falsely low faecal fat results (test now rarely done)


    ACTH, adrenocorticotrophic hormone; AST, aspartate transaminase; ALP, alkaline phosphatase; EDTA, ethylenediamine tetra-acetic acid; LDH, lactate dehydrogenase; PTH, parathyroid hormone; PSA, prostate-specific antigen.



  • correctly labelled specimens,


  • appropriate laboratory liaison,


  • speedy delivery to the laboratory.

Remember that treatment based on technically correct results from a wrongly labelled or collected specimen may be as dangerous as a faulty surgical procedure. ‘Unlikely’ results are checked in most laboratories to make sure that they have not been transposed with the results for another patient.

All patient samples are potentially infection risks. Informed consent may be needed for acquired immunodeficiency syndrome (AIDS) testing and also certain genetic tests. All blood specimens should be sent in leak-proof, sealed plastic bags, with the request form in a different pocket in the bag. Failure to comply with these guidelines may put many people, including porters and laboratory staff, at unnecessary risk.



REQUESTING PATIENT SAMPLES


Patient identification

Accurate and legibly written information about the patient is essential, although electronic requesting systems are now widespread in some areas. This information includes the patient’s:



  • hospital case number, and/or healthcare number,


  • surname and first name(s), correctly and consistently spelt,


  • date of birth, rather than age.

These would usually be considered the minimum acceptable dataset for patient identification details.

Any of these may be recorded inaccurately on the form and, unless there is complete agreement with previous details, results may be entered into the wrong patient’s record either on a computer or in the patient’s case notes, causing confusion and possible danger to the patient. The National Health Service (NHS) number is being used as a unique individual identifier in the UK.

It is important also to include relevant clinical details so as to facilitate correct interpretation of the results.


Location of the patient and identification of the clinician

It should be obvious that, if the ward or department is not stated, it may take time and effort to determine where the results should be sent. The requesting doctor must sign the form legibly, and also state how he or she can be notified rapidly, for example by ‘bleep number’, in case abnormal results requiring urgent action are found or advice needs to be sought about treatment. The doctor must check the completed request form to be sure that the information given is correct; it is also important to include his or her name and contact details.

Request forms designed by pathology and other departments ask only for information that is essential to ensure the most efficient possible service to the clinician and therefore to the patient. Now quite widespread is the use of electronic test requesting, which should improve patient identification and speed up the process and may replace ‘paper’ requests.

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Jun 30, 2016 | Posted by in BIOCHEMISTRY | Comments Off on Patient Sample Collection and Use of the Laboratory

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