Blood Test Interpretation Guide

Introduction

Interpreting blood tests is a core clinical skill. In this guide, we cover essential tips for blood test interpretation, including the basic principles of interpreting blood tests and identifying critical results.

Check you have the right patient

There is a reason why confirming patient ID is at the start of every OSCE checklist. Mistakes can easily occur when managing multiple patients on a busy ward round. It is vital to check the patient ID at the top of every blood test result (name, date of birth, and hospital number).

Be particularly vigilant with patients who share similar identifiers.

If someone shows you a set of results, do not assume they are correct for that patient. Other people make mistakes too! Always check for yourself. This should become second nature and something you do automatically whenever you encounter new results.

Check when the blood tests were performed

After checking the patient ID you should look to see when the blood test was taken. Are you looking at today’s result, or is this a result from last week or last month?

For some conditions and in high-dependency clinical areas, patients may have several tests a day, so ensure that you are looking at the latest results. 

You should also pay attention to when the blood test was reported as this may differ from when the sample was taken. This could be due to a delay in transporting the sample to the lab (samples from the community can spend several hours in transit) or in processing the results (blood cultures are routinely incubated for several days). 

It is also key to consider when the sample was taken in relation to the clinical context. For example, a troponin level should be interpreted in relation to the onset of chest pain and a paracetamol level relative to the time of ingestion. In these situations, you should look closely at the time the blood test was taken NOT the time it was reported. 

Consider the clinical context

Context is everything. Think of yourself as a detective trying to gather all the information before making a decision. A blood test in isolation means very little. It is useful to ask yourself these questions: who requested the blood test, why did they request it, and what is the patient like clinically?

Who requested the blood test?

Was this requested by the GP as part of a routine health check-up? Was this a specialist test requested during an outpatient appointment? Or is this a test requested by the inpatient team to monitor a specific condition?

Why was the blood test requested?

This is probably the most important question you will ask yourself when faced with a set of blood test results.

 Ideally, if you are checking blood results you did not request yourself, you should receive a handover explaining the clinical context. If you are unsure of the clinical context then have a look at the medical notes to find the indication. 

What does the patient look like clinically?

Remember to treat the patient, not the blood test. You should correlate any significant findings with the patient’s clinical state.

For example, a large drop in haemoglobin in a patient who looks well, with normal vitals and no signs of bleeding, is likely to be a spurious result (e.g. a sample taken from a drip arm). However, an increase in inflammatory markers in a patient who is tachycardic with a fever may indicate sepsis.

Reference ranges (not “normal ranges”)

Reference ranges guide clinicians in interpreting blood results. They establish parameters within which the blood results of most healthy patients (typically 95%) will fall.

However, health is a relative and not an absolute state. It is wrong to think of the reference range as a binary measure, where results that fall within the range are considered ‘normal‘ and indicative of health and those that fall outside of the range are ‘abnormal‘ and indicative of disease. 

A subset of healthy individuals will, by definition, have results which fall outside of the chosen confidence intervals. This becomes a significant number if you are looking at hundreds of results per day. 

Additionally, reference ranges do not always account for important variables such as age, ethnicity or sex. They are also not indicative of what is ‘normal’ for the patient in front of you.

Identify and act on critical results

The following pointers will help you identify which tests are critical and need actioning versus which ones can be filed away.

Look at the trend

Looking at the trend in the blood test over time is a good way of assessing the result’s significance. Click on the ‘graph’ button if that functionality is available for a graphical representation of the results.

A sudden change is much more likely to be of critical significance than a gradual one.

An eGFR of 25 in a patient with CKD stage IV is unlikely to be significant. However, the same result in a patient with baseline normal kidney function is a critical result which requires urgent investigation (are they dehydrated, septic or in shock? Is there an obstructive element? Do they need a catheter? etc.). 

Another commonly encountered abnormality is a raised white cell count (WCC). If you see a WCC of 17 x 10⁹/L (3.6 – 11.0 x 10⁹/L) in a patient on oral flucloxacillin for cellulitis, you need to know whether this is trending up or down. A down-trending WCC is reassuring, whereas an uptrending WCC should prompt a review.

You should check the patient’s clinical condition, whether or not they have grown any resistant organisms, and consider escalating to IV antibiotics.  

Decision protocols and escalation plans

Your hospital may have decision protocols and escalation plans in place for commonly encountered blood test abnormalities. These can help you decide what to do when you have abnormal blood test results.

Hyperkalaemia is a critical result that often has a hospital-wide management protocol. A typical hyperkalaemia management plan includes obtaining an urgent ECG (in all moderate to severe cases), giving IV calcium to protect the myocardium if ECG changes are present, administering potassium-lowering therapy, and identifying and treating the underlying cause. 

If there is no hospital policy to help guide you, you should see if the patient has any specific thresholds requiring action documented in their notes. For example, a patient on chemotherapy whose neutrophil count falls below 1.0 x 10^9/L may need discussion with a haematologist and initiation of treatment for neutropenic sepsis if signs or symptoms of infection are present.

Does the result make sense?

Any test result that is grossly outside of the expected reference range is likely to need urgent action. However, remember that the abnormal result may be due to artefact in some cases. 

This is often the case with blood results that are grossly abnormal but do not fit the clinical picture. Things to consider include: Has the sample been hemolysed? Was it taken from a drip arm? Was it taken from the wrong patient? Remember that a venous blood gas (VBG) is a quick and easy way of getting a repeat result for a variety of blood tests. 

Another tip is to look at what the blood results are telling you as a whole. An isolated result may not make much sense, but when considered in the context of the rest of the bloods, you may start to build a clinical picture.

For example, a raised urea in combination with a drop in haemoglobin may indicate an upper GI bleed and a raised ALP alongside a raised bilirubin and GGT indicates obstructive jaundice.

Common urgent actionable results

Common results that require immediate attention include hyperkalaemia, a sudden drop in haemoglobin, new AKI, raised troponin in the context of chest pain, and rising inflammatory markers. However, this is by no means an exhaustive list, and the significance of an abnormal result will ultimately depend on the clinical context.

Our reviewing and interpreting blood tests guide, which is part of the Geeky Medics preparation for clinical practice articles, gives more detail on some of the common blood test abnormalities.  Ensure you correlate the findings to the clinical situation and ask a senior for advice if you are unsure of what to do.

You are not alone in a hospital setting, and in many situations, it may be expected that you flag up significant findings to a senior colleague. 

Consider how the blood test helps inform the patient management

This is where your clinical reasoning skills come in. Clinicians must integrate information from various sources, including medical history, examination findings, imaging results, and blood tests, to formulate diagnoses and management plans.

When interpreting blood tests, ask why they were performed. If they were performed to monitor a condition, what does this reveal about the current management strategy? Are things improving, or is a change needed?

For diagnostic tests, consider false negatives and false positives and whether further tests are required to confirm the diagnosis, such as a CTPA for a suspected pulmonary embolism with a positive D-dimer.

Developing clinical reasoning skills takes time and practice, and it involves an understanding of medicine, psychology, and cognitive biases. However, it is also one of the most rewarding aspects of being a doctor!

Visit the Geeky Medics’ guides on clinical reasoning skills to find out more.

Documentation 

Make sure you document your findings and management plan in the patient’s notes. This is really important to ensure continuity of care.

If you are requesting further blood tests, ensure you either chase them yourself or hand them over for someone else to chase (be kind to your colleagues and give them a clear management plan explaining what to do with abnormal results). 

Specific blood test interpretation guides

For more information, read our specific blood test interpretation guides:

• Full blood count
• Urea & electrolytes
• Liver function tests
• Bone profile
• Coagulation screen
• Thyroid function tests
• Lipid profile
• Haematinics
• Hepatitis B serology
• Arterial blood gas
• Venous blood gas