Key Factors That can Affect Results

Common causes of spurious results due to incorrect collection or storage

Note: Common analytical interferences are screened for and if detected no results will be reported for that analyte.

If an analyte is clinically very important e.g. creatinine in a liver patient suspected of hepato-renal syndrome then contact the clinical team, who will advise on alternatives.
 

Problem

Cause(s)

Affects

Delay in processing

  • overnight storage
  • delay in transit

Increases potassium, & phosphate

Incorrect storage

  • storage of samples overnight in fridge

Increases potassium

Decreases bicarbonate

Haemolysis

  • Forcing blood through needle into tube
  • Difficult to bleed patients (e.g. paeds)
  • Storage frozen
  • Very delayed samples

Increases potassium, phosphate, bilirubin.

 

Also increases AST, LDH, CK and decreases Troponin T & glucose

Incorrect sample site

  • Sample taken from drip arm

Increased drip analytes e.g. glucose, potassium etc.

Dilutional effect lowers other analyte concentrations

Incorrect sample tube

  • Contamination of red/ yellow top tubes with blood containing EDTA (from purple top tube)

Increased potassium

Decreased calcium, alkaline phosphatase, iron, magnesium

Lipaemia (fatty sample)

  • Sample taken shortly after fatty meal

Decreases sodium

Affects other analytes if severe

Analytes requiring special conditions

Analyte

Sample Tube

Special Conditions

ACTH

Adult Purple Top Tube (EDTA)

Send to lab ASAP (within 15 mins). Transport on ice if possible.

Ammonia

Adult Purple Top Tube (EDTA) or Red Top (Paeds)

Send to lab ASAP (within 15 mins). Notify lab before taking. Avoid haemolysis.

Calcitonin

Adult Red or Yellow Top Tube

 

Send to lab ASAP (within 15 mins). Transport on ice if possible.

Glucose

Adult Grey Top Tube (Fluoride oxalate) or Yellow Top (Paeds)

Yellow/red top tubes are acceptable from A&E as processed within 2 hours

Gut hormone profile

Special green top tubes containing protein inhibitor

Before taking contact Biochemistry who will provide tubes and ice

Homocystine

Adult Red or Yellow Top Tube

Send to lab ASAP (within 15 mins). Transport on ice if possible.

Insulin/C-peptide

Adult Grey Top Tube (Fluoride oxalate) or Yellow Top (Paeds)

Send to lab ASAP (within 15 mins). Transport on ice if possible. Insulin is only sent for analysis if glucose < 2.5 mmol/L (unless specifically requested)

Lactate

Adult Grey Top Tube (Fluoride oxalate) or Yellow Top (Paeds)

Send to lab ASAP (within 15 mins). Notify lab before taking. Avoid haemolysis.

PTH

Purple Top Tube (EDTA)

Stable for 8 hours in EDTA

Renin

Adult Purple Top Tube (EDTA) or Red Top (Paeds)

Send to lab ASAP (within 15 mins). Do not use ice.

Take sample after 15 mins recumbency.

CSF Bilirubin (Xanthochromia)

1 mL (last sample) required

Serum required for bilirubin and total protein

Send to lab ASAP (within 15 mins). Protect from light. Do not use airtube.

 

Commonly Encountered Analytical Interferences (in Bold) or Biological Actions

Analyte

Drug or Substance Interfering

Effect

Alkaline Phosphatase

·      Anticonvulsants, barbiturates, oral contraceptive

·      EDTA contamination

­

Up

Down

Calcium

·      Prolonged tourniquet use, Vit D, bendrofluazide

·      EDTA or citrate contamination, citrated blood

­

Up

Down

 

Cholesterol

·      Oestrogens

Down

Creatinine

·      High bilirubin (icterus) and lipid (lipaemia)

Down

Gamma GT

·      Anticonvulsants, barbiturates, alcohol

­

Glucose

·      Frusemide, thiaxides, corticosteroids, stress

 

­

Up

Potassium

·      Insulin, corticosteroids, loop/thiazide diuretics

·      K+ sparing diuretics, ACE inhibitors

·      Haemolysis, EDTA contamination, excess storage

Down

Up

Up

­

­

Prolactin

·      Oestrogens, MAO inhibitors, cimetidine

Up

Sodium

·      Lithium

·      Diuretics, carbamazepine, fluoxetine, lipaemia

­

Up

Down

Thyroxine

·      Amiodarone, pregnancy

·      Oestrogens

·      Phenytoin, corticosteroids, heterophilic antibodies

Up or Down

­

Up

Down

 

blood sciences

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