Common Blood Tests and Other Pathology Tests in ICU

People who are seriously ill and admitted into the intensive care unit (ICU) will have a wide variety of tests done on different body fluids, such as blood and urine. This is to help diagnose potential medical problems or to see how well their treatment is working.

Why do people in intensive care have lots of blood tests?

People in intensive care are usually seriously ill and their condition changes often. For this reason, they will have a wide variety of tests done on different body fluids, such as blood and urine. This is to help diagnose potential medical problems or to see how well their treatment is working.

Why so many different tests and so often?

The types of tests and how often they are done depend on why the person is in ICU and how stable they are. Most often, testing happens when the person is first admitted to ICU, and then every day. However, in a number of different situations, some tests will be repeated quite often.

How are the tests done?

Most tests are done in the same way as an outpatient pathology service – that is, blood is withdrawn from a vein in the arm. However, because blood tests need to be done more often in ICU, a thin catheter is inserted into an artery. This catheter is called an ‘arterial line’ and it stays there to help minimise their discomfort from repeated needles. But, unfortunately, some tests cannot be done using an arterial line.

All staff who withdraw blood for these tests are trained to minimise the person’s discomfort.

Who will explain the results?

Usually the tests are explained generally, such as whether the results were normal or abnormal.

It is probably not helpful for the person in ICU and their loved ones to become anxious about the day-to-day results of most blood tests. This is because a proper understanding of the results is difficult without a background in medicine or the health sciences.

However, if the results are of serious concern, such as anaemia or worsening kidney function, they will usually be explained.

If you have further questions, please ask the nurses and doctors caring for the person in ICU .

Blood tests

Electrolytes (sodium, potassium and chloride), urea, creatinine (EUC)

EUC involves taking between 1-4 millilitres (mL) of blood from either a vein or an artery. This test looks at the basic chemical balance of the blood, as well as kidney function. An imbalance in the chemical makeup of the blood can happen in a number of medical conditions.

This test will also show whether a body salt needs to be replaced. And urea and creatinine levels show how well the kidneys are working.

EUC is usually done when a person is admitted to ICU , and then regularly, which could be daily. If there is a concern about kidney function or chemical balance, then EUC will be repeated more often. For example, if a person is on dialysis, this test is usually done four times a day. If potassium (an essential body salt) is being replaced, the test may be repeated to see if enough has been given. The results are usually available within 1-2 hours.

Arterial blood gas (ABG)

This test involves taking a small amount of blood, around 1-2 mL from an artery. ABG is extremely important for several reasons, such as:

  • it tells the intensive care staff how well the lungs are functioning (showing the oxygen and carbon dioxide levels)
  • it will show whether or not the respiratory (breathing) support the person is receiving is enough
  • it gives an idea of the metabolic balance in the body.

Examples of conditions where ABG is important could include pneumonia, poorly controlled diabetes, cardiac failure and kidney failure.

Most people admitted to ICU will need at least one ABG , usually upon admission. But this test may be done more often than EUC , especially if the lung function is weakened and/or the person is receiving respiratory support.

ABG is done using a smaller needle than other blood tests, but blood must be taken from an artery. The most common arteries used are those in the patient’s wrists because these arteries are easy to get to. Some bruising may result after this test is done. The results are usually available in a short time.

Liver function tests (LFTs)

The same blood sample taken for a EUC test can be used for LFTs . As the name suggests, LFTs check how well the liver is working. The liver cleanses the blood of waste products, but can function poorly if the person is sick. LFTs are usually done daily, with results available in 1-2 hours.

Full blood count (FBC)

This blood test involves taking approximately 4 mL of blood from a vein or artery. As the name suggests, an FBC ‘counts’ the number of each of the different blood cells in the blood.

Haemoglobin or ‘Hb’ shows those parts of the blood cells that carry oxygen around the body. A low Hb is called anaemia. A blood transfusion may be required if the Hb is too low.

The white cell count (WCC) shows there is an infection or inflammation in the body. And platelets form an essential part of the blood clotting process.

An FBC is usually done upon admission and then daily. If blood transfusions are needed or there is a concern about bleeding, the FBC will be done more often.

Coagulation studies (coags)

This test checks whether the blood is clotting as it should. If it is not, then the person can bleed to easily.

Blood clotting is a complex process that can be affected by a number of serious illnesses.

Around 4 mL of blood from a vein or artery is needed to run this test. Coags are done upon admission, and are repeated regularly if the coagulation status is abnormal or the person is receiving medicines that affect it.

Regular coagulation studies are needed when a person is receiving anti-clotting medicines such as heparin or warfarin.

Blood sugar level (BSL)

A BSL can be done in different ways. The blood may be taken from a sample used for a EUC or an ABG, or by a finger prick. If a blood sample is taken through a finger prick, it is generally done at the person’s bedside using a small glucometer (a device that measures blood sugar levels).

BSL is an important blood test. Unless there is a concern over the level of sugar in the blood, BSL is usually done 1-2 times a day. But it will be done more often if the:

  • person is diabetic
  • BSL is unstable
  • person is receiving a medicine that could affect their BSL
  • BSL needs to be kept within a tight range.

Other tests


A urinalysis is a routine test that’s done by the nursing staff in intensive care. It involves inserting a test strip into a urine sample. This strip can show a number of problems, including infection, dehydration, abnormal kidney or liver function, and/or presence of glucose or ketones or blood in the urine.

Pathology tests to identify possible infection

Pathology tests are usually done if an infection is suspected. It normally involves testing the blood, mucus and urine, as well as testing the samples from any other sites, such as wounds.

Doing pathology tests to check for infection is called a ‘septic workup’, and is usually done on people with a high or increasing temperature, or those who have been in ICU for a long time.

Blood cultures

Blood cultures are done if a person shows symptoms of an infection, such as a raised temperature, a low blood pressure or a wound that appears infected, and to check how well antibiotic treatment is working.

Usually two sets of tests will be done, followed by further tests if needed. Some ICUs do routine blood cultures on people who’ve been in ICU for a long time to pick up possible infections early.

Blood cultures involve taking 10-20 ml of blood from a vein under sterile conditions. This blood is placed into specific sample bottles and sent to the lab. At the lab, small amounts of blood from the sample are placed onto special plates to see what bugs may be growing in the blood.

An initial result will be available within several hours, but the final results from a blood culture can take several days. If a person is quite sick with a suspected infection, antibiotics will be started before the full test results are available. This antibiotic treatment may then be changed if needed when the full results are available. Unfortunately, it is not always possible to work out the cause of the infection.

Mid stream urine test (MSU)

MSU involves taking a small urine sample under as clean conditions as possible. Since most seriously ill people have a urinary catheter in place, obtaining a clean sample is quite easy.

The urine sample is taken to the microbiology lab and tested to check for infection. The results are usually available within 1-2 days. If it is suspected that the renal (kidney) system is the cause of a possible infection, then antibiotics that can kill a wide range of bacteria will be started. Once the bug is identified, a more specific antibiotic can be given.

Sputum (mucus) testing

The lungs are quite often the source of an infection in a person admitted to ICU. It is normal to have a number of bacteria in the throat and upper airways, so their presence in the mucus of the lungs does not necessarily mean that the person has pneumonia.

A  sputum sample (mucus specimen) can be obtained in a number of ways. Ideally, the sputum should be taken from deep inside the lungs, so simply coughing the sputum into a sample container is not always ideal. Usually, if the person is ventilated, the mucus will simply be sucked out with a clean suction catheter.

To get the best sample, a bronchoscopy (where a thin tube is inserted into the lungs) may need to be done. Or a simpler procedure called a ‘non-bronchoscopic bronchi alveolar lavage’ (NB-BAL) may be used.

In both bronchoscopy and NB-BAL, 20-40 mL of saline is injected into the lower airways and immediately suctioned out. This ‘washes’ out the lower airways and returns a cleaner sample for examining.

Wound swabs

If a wound, such as a graze or surgical cut, appears infected, a wound swab is usually done. This involves cleaning the wound with normal saline and ‘wiping’ a sterile swab across the wound. This swab looks much like a cotton wool bud, but with a longer handle. The swab is taken to the microbiology lab for testing. Results take several days.

Occasionally a wound may be opened because there may be a large amount of swelling and/or discharge. Surface wounds generally don’t need antibiotic treatment. If the wound is considered to be causing a widespread infection, antibiotic treatment may be started.

Cerebrospinal fluid (CSF)

CSF is the fluid that bathes the brain and spinal cord.

A sample of this fluid can be taken in different ways. A lumbar puncture may be performed. This procedure involves inserting a needle between two vertebrae (bones in the spine) in the lower back under sterile conditions. The needle goes into the subarachnoid space in the brain, and a small amount of CSF is drained and sent to the lab to check for infection and other abnormalities. A lumbar puncture may be performed in intensive care if infection is suspected.

Another way to get a CSF sample is if the patient already has in place an intraventricular drain (a plastic tube placed by neurosurgeons to drain extra CSF to relive pressure on the brain). A sample can be taken from this drain and sent to the lab to check for possible infection as a routine procedure.

Other body fluids

Samples of fluids from other parts of the body may be sent for testing if they could be the source of infection. If the person has a drain tube (e.g. intercostal catheter) in place, a sample from this site can easily be taken for testing.

Publication details

Common blood tests and other pathology, version 1.1, 2015. Kaye Rolls CPO, ICCMU.

