When the afternoon comes, you will often find yourself going through all the blood results for the day and wondering what you need to do when they're abnormal, whether to put them out for the next day and which  Here we will discuss all of that!

Firstly, remember you should aim to do investigations that will change your management & if you request a test, you are responsible for making sure it is reviewed or handed over.

Secondly, there are no hard rules on who does or doesn't need blood tests and when & how to respond - these come with clinical experience, so as always see the information below as a guide.

When should I request routine blood tests?
Routine bloods usually means FBC, U&Es & CRP. For some, this may also include LFTs - particularly gastroenterology & general surgical patients.

If your patient is medically unwell and you're not sure which way things are heading, it is often worth repeating these daily. The aim is to give support to the clinical situation looking for common things that might change on a daily basis: bleeding, AKI or infection - hence they might be helpful. Occasionally, the results will be completely unexpected and might alert you slightly earlier to a patient at risk of becoming unwell.

If the results are trending in the wrong direction, often bloods are repeated daily. If they are trending in the right direction, the frequency may be reduced to every other day.

When should I NOT request routine blood tests?
Patients are often frustrated by daily blood tests which frequently won't really change your management. It is tempting though to request bloods on everyone just in case they worsen or in case your seniors want it to avoid you having to bleed the patient later. However, having to check or handover these bloods increases your workload and it is at an unnecessary cost to the NHS. Handing over a lot of unnecessary bloods that haven't come back is also really difficult for the evening or night team.

Therefore, aim to avoid daily blood tests in those who are medically fit for discharge or are stable. Once weekly is usually enough. Ask each time what you're looking for & how it will help support your management plan.

Aim to avoid blood tests on the weekend unless they're critical to their weekend care. If it is, explain this to your patient so they don't refuse it. Otherwise, the weekend team has to bleed the patient when they find out quite late that the sample was never taken.

How do I interpret them? When is it urgent?

Full blood count
  • A haemoglobin drop is usually concerning. Small fluctuations (less than 5g/L) can occur with the hydration status. A corresponding rise in urea suggesting an Upper GI bleed would be more concerning. Don't forget haematinics to review if there's a reversible cause of chronic anaemia. 
  • Neutrophils above the normal range or below 1.0 are worth a second thought. 
    • The former might suggest infection so consider the CRP, temperature and whether the patient was unwell on review. Steroids can also increase the neutrophil count. 
    • The latter may suggest bone marrow suppression or a patient at risk of neutropenic complications like sepsis. Those of African descent do tend to have lower white cells and neutrophils which can be benign
Urea & Electrolytes
  • Electrolyte disturbances are common and usually transient. Do consider whether treatment is indicated or whether it can be simply repeated the next day (if only just outside normal range).
  • Urea & creatinine are of great use to diagnose AKI (as a dropping urine output is frequently unnoticed)
  • Small fluctuations are common due to the number of drugs that impact the liver in some way
  • Mild increases just above the normal range are usually not of clinical significance. These can be monitored for resolution. 
  • Significantly raised Bilirubin or deranged LFTs more than twice normal range or when the synthetic function is impaired (INR, albumin) that is far more concerning. 
  • Remember this often lags compared to the white cell count
  • Rising CRPs are usually an indication of some inflammatory process that requires the patient to be reviewed and considered for a septic screen
What are some common blood tests I might order?

Clinical Scenario Blood Tests (Routine plus...) Rationale
Tachycardia Magnesium & bone profile
Thyroid function tests (TFTs)
These are other causes that might contribute to the tachycardia
Chronic anaemia Blood film, B12, folate, iron studies (Transferrin, Ferritin, TIBC) Haematinic studies to identify the cause
Delirium Bone profile, LFTs, TFTs, haematinics Common causes of confusion in the elderly
Deranged LFTs Viral hepatitis screen (Hepatitis B surface Ag, surface Ab and core Ab & Hepatitis C IgG)
Full LFTs (AST & GGT)
Clotting & albumin
Viral hepatitis is a common cause. GGT and AST can support alcohol-related causes.
Clotting, albumin (& platelets) can support a diagnosis of chronic liver disease (in the right context)
Infection/Sepsis LFTs & venous blood gas Lactate is of value
Seizures/Fits Magnesium & bone profile Electrolyte derangement can cause seizures
Abdominal Pain Pregnancy test, bone profile, venous blood gas (glucose & lactate), lipase/amylase, LFTs, LDH (if pancreatitis suspected)
Consider repeating LFTs daily if biliary cause is being considered until resolved
Wide differential for abdominal pain is important. For biliary causes, it is important to monitor for biliary obstruction

Further reading
Written by Dr Akash Doshi CT2