30. Sampling body fluids and their preservation

5/12/2025

Why are body fluids sampled?

• To provide patients with a diagnosis

• To monitor patient compliance

• To determine a prognosis

• To monitor the effectivity of a treatment

What is the pathway of a sample?

• Pre-analytical

• Analytical

• Post-Analytical

What happens during the pre-analytical stage of the sample pathway?

• Collection of the sample (venepuncture)

• Labelling

• Transport

What happens during the analytical stage of the sample pathway?

• Diagnostic testing

• This is after the lab has received the sample and has checked that they have the correct sample and everything has been labelled correctly

What happens during the post-analytical stage of the sample pathway?

• Result is reported

• Samples are stored

Name 4 common sites for phlebotomy

• Median cubital vein

• Back of Hand

• Finger prick

• Heel prick

What is the process of sample collection?

• Patient identification

• Request form

• Select site for venepuncture

• Label tubes

• Transport samples

  • If urgent then doesn’t go with normal courier

• Minimum identifiers

What are the minimum identifiers of a sample?

• Full name

• Date of birth

• NHS number

What is plasma?

• The top layer of a whole blood sample after it has been treated with anticoagulants and centrifuged

• Still contains clotting factors

What is serum?

• The top layer of a whole blood sample that has been allowed to coagulate, after it has been centrifuged

• Contains very low to no clotting factors especially fibrinogen because it has been used up by the blood

Yellow top tube

Serum separator tube

Additive of SST

• An inert gel denser than serum but less dense than erythrocytes

• Coated with silica particles which encourages clotting

What is the yellow top tube used for ?

• Routine biochemistry

• Urea and electrolytes, C-reactive protein, liver function, creatinine, lipid profile, bone profile

• Needs to be inverted 5-6 times because contains silica particles

Polycythaemia

When SST is centrifuged and there are more erythrocytes than serum

What are the problems with a yellow top tube?

• Incomplete barrier formation after centrifugation

  • Gel may have not moved to where it needs to be

  • Gel may not have moved at all

• Sampling probe can get clogged if sample volume is too low

• Gel can sometimes interfere with drugs/ endocrine tests

Red top tube

• Either no additive or coated with silica particles

• Serum is obtained after centrifugation

Uses of red top tube

Measurement of analytes where the yellow top cannot be used due to interference of the gel

• immunology tests

What are the two type of green top tubes?

• Light green top

• Dark green top

Additive of the light green top tube

• Gel

• Lithium heparin as an anticoagulant

Use of light green top tube

• Collecting plasma (NOT SERUM)

• Need to invert 8-10 times

Additive of dark green top tube

Just lithium heparin

Uses of dark green top tube

• Plasma needs to be removed after centrifugation

• Measure analytes: Ammonia, insulin, aluminium

Purple top tube additive

K-EDTA

Purple top tube uses

• Whole blood for FBC

• HbA1c- measuring glycaemic control for diabetes

• Looking at immunosuppressant drugs like cyclosporin

• Venous carboxyhaemoglobin

• Needs to be inverted 8-10 times

Grey top tube additive

• Potassium oxalate- anticoagulant preventing blood clotting

• Sodium fluoride - inhibits glycolysis in macrophages

Why do we need to inhibit glycolysis in a blood sample to get an accurate blood glucose reading?

• Without sodium fluoride the cells within the sample will metabolise glucose

• Giving an artificially low glucose reading

Uses of grey top tube

• Fasting glucose

• Lactate

• Invert 8-10 times

Dark blue top tube

• Purified glass tube containing no metals

Uses of dark blue top tube

Measuring:

• Copper

• Zinc

• Mercury selenium

Purpose of the order of draw?

• To minimise the risk of cross contamination or carry over of additives between different sample collection tubes.

• It is important because different additives can alter the result of the intended test if carry over occurs.

Order of draw

• Light blue

• Red

• Yellow

• Green

• Purple

• Pink

• Blue

• Gray

Mnemonic to remember order of draw

Lazy rabbits yawn gratefully preferring pink blankets generally

What causes haemolysed samples?

• shaking/inverting vigorously or expelling blood through a needle

• Hb is released from the erythrocytes into the plasma/ serum

Why can’t haemolysed samples be used?

• They interfere with calorimetric assays

• They falsely elevate potassium, phosphate, AST, LDH and magnesium

What causes icteric samples?

• High levels of bilirubin often due to in vivo haemolysis or liver disease

• They look dark brown- green in colour

Why can’t icteric samples be used?

They interfere with enzymatic assays

What causes lipaemic samples?

• Excess lipids in blood

• Sample taken after a short time or after ingestion of a fatty meal

Why can’t lipaemic samples be used?

They interfere with most assays

What is auditing?

Observing various processes and determining how well the processes are working and identifying areas of improvement

State the importance of sample auditing

• Part of quality assurance to ensure a high quality service is being delivered

• Introduces new improvements to minimise errors that were common in the past

What is the minimum sample acceptance criteria

• Minimum number of matching identifiers (OFTEN 3)

• Correct sample tubes for tests requested (colour top)

• Adequate sample volume

• Reject the sample if necessary

• report rejected sample(s)

What are the 2 types of urine that are sent into biochemistry?

• 24 hour urine (collecting urine over 24hrs)

• Random urine

What are the analytes that can be measured from a 24hr urine sample?

• Urine electrolytes

• Creatinine

• Copper

• Albumin

• Cortisol

• Protein

Which analytes can be measured from a random urine sample?

• Bence Jones proteins

• Osmolality

• Albumin: Creatinine ratio

What are sweat samples used to determine?

Chloride concentration

What can be diagnosed from sweat samples?

• Cystic fribrosis

• CFTR mutation screen

Why do we get carry over?

• Backflow of sample in vacutainers

• Pouring one specimen into another

What are UN numbers?

One of four United Nations number assigned to a sample dependent on their risk categories

What is the pneumatic air tube system/ POD system?

• Transport system for samples

• Enter the location code and the pod is sent to the lab

• Red pods for urgent samples

• Blue pods for routine

Why can’t histopathology and CSF samples be transported by the POD system?

• They are precious samples

• Formalin fixed histopathology specimens- if the specimen pot breaks formalin will cover the system