In the following article, we will look at factors which can invalidate blood test results, and the ways in which these tests might be affected.

Haematology and biochemistry result errors are seldom due to the laboratory analysers – most are due to factors which occur between the sample being taken and the sample being processed. These are predominantly –

• Time delay in transporting the sample to the laboratory – delays of more than 24 hours between sampling and analysis can invalidate results, with biochemical tests being more vulnerable to inaccuracies.
• Storage conditions – samples should be refrigerated at 4°C.

Generally, the longer a sample is kept at room temperature, and the longer the time delay before a sample is separated [spun to separate into its components] and analysed, the greater the potential for test errors.

How Are Blood Tests Affected by Storage Changes? - looking at tests most requested by Large Case Underwriters:

1.      Full Blood Count (FBC) – the FBC includes red cells, white cells [and subtypes], platelets, Hb and associated factors. With storage or delay in processing, there can be clumping or clotting in the sample, causing the counts to be spuriously low. After about 24 hours haemolysis [RBC breakdown] sets in and RBC falls, while MCV [mean cell volume] tends to rise.

• WBC [all subtypes] – Little Effect/Relatively Stable
• Platelets – Little Effect/Relatively Stable
• RBC – Falls
• MCV – Rises

2.      Blood Glucose – glucose levels in whole blood samples can fall at up to 0.4 mmol/L per hour. Beware, that in diabetics this could result in a spuriously normal/near normal result [pseudo-normalisation] – so check the glycosylated haemoglobin [HbA1c].

• Blood Glucose - Falls

3.      Glycosylated Haemoglobin (HbA1c) - this is an important index of blood sugar control. It is used as a measure of glucose control in diabetics, but can also be used as a marker for the presence of diabetes. Storage at room temperature, and delay in processing, can cause a rise in the HbA1c, which in a non-diabetic might suggest diabetes.

• HbA1c – Possible Rise

4.      Electrolytes and Urea - these results are susceptible to invalidation by problems with sampling (“traumatic” phlebotomy), duration of storage at room temperature and time taken to sample separation. Increased permeability of post-phlebotomy blood cells can significantly change, causing:

• Sodium - Falls
• Bicarbonate - Falls
• Uric Acid - Falls
• Potassium – Rises
• Phosphate – Rises after 24 hours
• Total bilirubin – Rises with storage-related haemolysis, Falls with sunlight exposure
• Chloride- Largely Stable
• Urea – Largely Stable
• Creatinine – Largely Stable

5.      Liver Function Tests - a wide range of tests collated as ‘liver function tests’ representing the synthetic and excretory functions of the liver.

The following remain stable, with the proviso of evaporation where all levels can rise:

• Alanine transaminase [ALT] - Stable
• Aspartate aminotransferase [AST] - Stable
• Gamma glutamyl transferase [GGT] - Stable
• Albumin – Stable
• Alkaline Phosphatase [ALP] – Stable [Be aware that ALP is also found in kidney, bone and intestinal mucosa]

6.      Lipid Studies -

• Total Cholesterol – Stable [for up to 7 days if stored in a fridge]
• Triglycerides [TGs] - can Rise after several days at room temperature
• High Density Lipoprotein [HDL] - can Rise after several days at room temperature

7.      B-type natriuretic peptides – BNP and NT-proBNP – are released from cardiac muscle cells when the heart is strained by pressure and volume overload. BNP is the active hormone, and NT-proBNP is an inactive peptide precursor.

BNP causes mild vasodilation and acts on the kidneys to cause sodium and fluid loss – the net effect is to reduce pressure on the heart. Elevated BNP levels are associated with heart failure - the higher the levels, the worse the heart failure.

BNP and NT-proBNP are released in equal amounts. The estimated half-life of BNP is 20 minutes but that of NT-proBNP 120 minutes. This explains why NT-proBNP serum levels are approximately six-times higher than BNP.

NT-proBNP is more stable than BNP and is a better marker to measure.

• BNP - should ideally be collected and tested within 4 hours. Samples can be stored and tested for up to 24 hours if prepared and then stored at 2-8°C
• NT-proBNP, by contrast, can be valid for testing if appropriately stored and refrigerated for up to 3 days. Levels will Fall with storage

8.      Prostate-Specific Antigen (PSA) - is a glycoprotein produced by normal and cancerous prostate cells. Although PSA is commonly used as a screening measurement for prostate cancer, it is not a perfect tool. The amount of PSA produced by cancer cells is variable. Advanced or poorly differentiated prostate cancers can produce abnormally low amounts of PSA, and so produce relatively normal readings.

Other factors that can raise PSA levels include digital rectal examination, prostatitis and benign prostatic hyperplasia.

• If stored at 20-25°C, PSA can Fall by 25% in 1 week
• If stored at 5-8°C, PSA will remain Stable for 5 days

Summary and Recommendation

The longer a blood sample is kept at room temperature, and the longer the delay before it is separated and processed, the greater the likelihood of potential errors. When interpreting any abnormal results, it is important to note the time and date for sampling, and laboratory processing, to see if there has been a significant delay. Samples which have been delayed for more than 24 hours from being taken, to being processed, may have limited value for biochemical assay. FBC samples are relatively more robust.

The guidance above may help determine whether abnormal results are due to storage changes. If questions remain, then advice from an experienced Chief Medical Officer might avoid the need to repeat the test.

Author: Dr Debesh Mukherjee & Dr Peter O’Connell, Company Medical Advisers Ltd.
Contact: Admin@companymedicaladvisers.co.uk
 

Last updated: 03/10/2024
References: available on request.