Reviewing and Interpreting Blood Tests

Introduction

Reviewing and interpreting blood tests is a core skill for all doctors, however it is a skill that takes some time to develop. 

This article is not a complete guide to each blood test and how to manage each specific abnormality but aims to give an overview of how to approach blood test interpretation­­­.

For more information on specific tests, check out our guides to the full blood count, urea & electrolytes, liver function tests and the bone profile! We also have a full article on managing electrolyte disturbances.

General principles

Normal working hours

In a medical ward setting, the doctors will generally request the next day’s blood tests for their patients at the end of their shift. These bloods are commonly taken early in the morning by a phlebotomy team, and results will become available in the late morning or afternoon.

Blood tests taken later in the day (e.g. if a patient becomes acutely unwell) may need to be handed over to the on-call team (see below).

As a doctor on the medical team, one of your key responsibilities is to review the blood tests of all patients under your care and act upon these. These actions are often simple management tasks such as prescribing electrolyte replacement or IV fluid regimens.

It is also important to escalate any results of concern to your senior doctors. Each ward team will run slightly differently, but on most wards, the senior doctors will set aside time to sit with the juniors and review results. This is sometimes known as a ‘bloods round‘. If this is not the case, the senior medical team will be expected to be informed about any results of concern.

On call

During an on-call shift, reviewing the blood results of multiple patients you may not be familiar with is common. This can happen by ‘chasing’ blood results handed over by the previous team or with critical results phoned through to you from the hospital laboratory.

It takes significant clinical experience to know which abnormalities must be acted upon during the on-call period. By the end of your first year on the wards, much of this will be second nature; however, these decisions can be cognitively taxing in the first few months. Always escalate to a senior clinician if you have any doubts.

All doctors will remember their first shifts, and the struggle to know the significance of some borderline blood results, so never be afraid to escalate if unsure. These conversations can be great learning opportunities!

‘Chasing’ blood results

When taking handover at the start of the shift and asked to ‘chase;’ results, always try to get as much clinical information as possible:

• An overview of the patient and their clinical state
• Why the blood tests were taken (e.g. monitoring serum potassium levels after treatment)
• What the team would like done in response to the result (e.g. further insulin/dextrose therapy if the potassium remains >6mmol/L)

It is not always possible to predict every result. However, having a general idea of what the medical team expects in response to the results can be very helpful when you get around to seeing the results (which may be many hours later, with many competing on-call demands).

Results phoned through

If results are phoned through to you from the reporting laboratory, the result generally meets a threshold for significance and will need to be acted upon! You may not have had a handover about these patients, so they will require a more thorough assessment.

Where there is time (i.e. not a time-critical result such as hyperkalaemia), always start by reading the patient’s notes to put the result in clinical context. If the patient is clinically well, there is far less urgency to act upon abnormal blood test results, particularly if there are only mild derangements.

After your assessment, results should be much easier to interpret with the patient’s clinical context in mind.

Always remember that venous blood gas can give you rapid results for various parameters (e.g. haemoglobin, potassium) and can also provide key indicators of physiological distress (e.g. acidosis, raised lactate). This should generally always be included when repeating samples suspected of being spurious. If the result is confirmed from a VBG from a separate blood sample, it should be treated as accurate.

As always in medicine, the trend of results is very important. A result slowly trending in one direction (e.g. slowly down-trending haemoglobin) is generally less urgent than results with significant new derangements.

This is particularly true in electrolyte disturbances. Having said this, the patient’s normal medical team may have overlooked a slowly trending result, and it may pass a threshold that needs acting upon on the results you see during the on-call shift (e.g. Hb <70g/L may need a blood transfusion). These decisions can be more subtle and should be escalated to seniors if they are concerned.

Common blood test abnormalities

This section covers some key considerations when managing abnormal blood tests in practice. Remember, we have a dedicated guide to electrolyte disturbances, which will be some of the most common abnormalities seen! Other common abnormalities are discussed below.

Full blood count (FBC)

Low haemoglobin (anaemia)

Consider the trend. Is this an acute drop or a slow downtrend?

• In an acute drop, assess the patient for sources of bleeding and treat as appropriate (e.g. gastroenterology / surgical referral).

If there is no clear source of bleeding and the patient is well, consider repeating the sample (alongside VBG and group & save). If the result is not spurious, discuss the next investigation steps with a senior (e.g. further imaging to assess for hidden bleeding, haemolysis screen, etc.).

• In a slow downtrend, does the patient have symptoms of anaemia needing transfusion? Do they have a pre-defined transfusion threshold?

Raised white cell count (WCC)

A raised white cell count is generally a marker of infection/inflammation but is relatively non-specific – remember, not all patients with raised WCC have infection.

This should be interpreted within the clinical context of the patient. Any patient with raised inflammatory markers such as WCC and CRP should have an examination to look for sources of potential infection and consider a ‘septic screen’ (urine MCS, chest X-ray).

If the patient is systemically well, without obvious signs of infection / other pathology on examination, this may need no further acute management.

If you are concerned that the WCC is excessively high (e.g. > 35 x 10⁹/L) and could represent a non-infectious process (e.g. malignancy), discuss with a senior. This will unlikely require management during the on-call period but will require close follow-up.

Neutropenia

Neutropenia can occur in the context of chemotherapy or other haematological disturbances and puts patients at higher risk of serious infective consequences, particularly when neutrophils are <0.5 x 10⁹/L.

If a patient becomes newly neutropenic, it is important to alert the nursing staff to move them to an appropriate room and start neutropenic barrier precautions to lower the risk of infection.

Be aware that fever in a neutropenic patient is a medical emergency and should always be treated as neutropenic sepsis in the first instance.

Thrombocytopenia

A reduced platelet count can occur for many reasons in a hospitalised patient and may not need urgent intervention unless severe or the patient is bleeding.

Again, the trend is very important here. Patients with known thrombocytopenia (e.g. secondary to a myelodysplastic syndrome) can chronically have very low platelets with low transfusion targets in the absence of bleeding (e.g. transfuse if platelets <10 x10⁹/L), thus may not need any intervention during the on-call period.

In patients without a known haematological disturbance, an acute drop in platelet count is of more concern and should be discussed with a senior doctor. Most cases can be monitored in the first instance, particularly where the platelet count is >100 x10⁹/L.

When platelets are <50 x10⁹/L, anticoagulation (e.g. LMWH DVT prophylaxis) should be reviewed

If the patient is bleeding, any level of thrombocytopenia may require urgent platelet transfusion and should be discussed with haematology.

Urea & electrolytes (U&Es)

Raised creatinine

Always assess the trend of creatinine, as sometimes patients with severe chronic kidney disease (CKD) can have very high creatinine at baseline.

If there is a significant recent increase in serum creatinine (e.g. acute kidney injury), assess for the underlying cause of kidney injury (pre-renal, renal, post-renal) and treat as appropriate.

• Most cases of AKI in-hospital are pre-renal and will require IV fluid hydration as a first-line treatment
• Always perform a bladder scan to rule out urinary retention
• Remember to withhold any nephrotoxic medications and ensure strict monitoring of fluid intake and urine output
• Discuss with your seniors whether further renal imaging is required

Always assess the remainder of the electrolytes in a patient with AKI / CKD to ensure no developing hyperkalaemia or other electrolyte disturbance that may need urgent treatment.

Liver function tests (LFTs)

Mild liver function test derangement is common in hospital, and often will not require acute management during the on-call period in a well-patient.

In acute and severe LFT derangement, always assess the patient to ensure no signs of acute liver failure, particularly if the bilirubin is elevated.

Abdominal pain with associated LFT derangement can be a sign of surgical hepato-biliary pathology that may require urgent imaging and further investigation.

Review the medication chart to ensure there are no culprits for liver toxicity (e.g., supratherapeutic paracetamol dosing, new antibiotics).

Check coagulation studies, if not already performed, to assess the synthetic function of the liver.

Always discuss with a senior if unsure regarding the level of further investigation or management required.

Raised CRP

A raised CRP should be interpreted similarly to a raised WCC above – it is a non-specific marker of infection/inflammation.

• If the patient is well, this may need no specific action (consider a septic screen).
• If the patient is unwell or spiking fevers, they will need assessment as to whether a previously unknown source of infection requires treatment.

It is important to remember that the CRP can ‘lag’ behind the clinical state by up to 48 hours, so it can continue to increase even as the patient’s clinical status improves.

Raised troponin

These patients should be prioritised, particularly if you have no prior information (i.e. during an on-call shift). Always ask the nurses to perform an ECG and baseline observations before your review. Ask about chest pain (raised serum troponin in the presence of chest pain is an emergency requiring urgent management!).

Generally, if sending a serum troponin, the medical team should hand over to the on-call team with clinical context as to why the test is being sent and what action they would expect if raised (e.g. discuss with cardiology if troponin is rising).

A ‘serial’ troponin measurement in a patient with a known myocardial infarct is less critical, particularly if it is decreasing and the patient has no chest pain.

If the patient has a chronically elevated troponin, consider non-cardiac causes (e.g. CKD). It is still important to perform a baseline ECG and assess the patient for chest pain.

A raised serum troponin is a significant finding and should always be discussed with senior doctors, even if it seems benign.

Blood cultures

It is common for the laboratory to phone through positive blood culture results (e.g. ‘Mr. JB has a gram-positive coccus in both blood culture bottles’).

These calls can initially seem overwhelming (as you try to think back to medical school microbiology lectures!). A positive blood culture (bacteraemia) is always significant and should be discussed with senior doctors to ensure the patient is on appropriate antibiotic therapy.

It is important to first gather more clinical information on the patient, particularly:

• Whether they have a known infection/bacteraemia
• Whether they are systemically well or spiking fevers
• Current antibiotic therapy
• Any previous resistant organisms or positive cultures
• The presence of any indwelling lines or mechanical heart valves

This information will allow a reasoned decision about further management and rational antibiotic choice.

INR

Interpretation of INR results can be variable depending on a range of patient-specific factors.

For a brief overview, see our guide to after-hours warfarin prescribing in our common on-call jobs article.