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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