Making sense of blood tests, scans and x-rays

Whenever you see your doctor or visit the hospital you may have various tests, scans and measurements taken. It is an unusual visit if you do not have at least a blood test. What do these tests mean and why does your doctor do them? Are they all really necessary and is there any need for you to know the results as well as your doctor?

When you go for an outpatient clinic visit it is customary to check the following measurements height, weight, blood pressure and heart rate, respiratory rate, and the level of oxygen in the blood or oxygen saturation. Lets start by discussing them.

Height and weight are obviously important especially in children and teenagers who are still growing. Some of you may not know however that teenagers with sickle cell disease often carry on getting taller for several years after their contemporaries have stopped growing. You should try and track your weight and, if you are still growing, ask your doctor to show you your height and weight plotted on a growth chart, which compares your values with other individuals of the same age.

Male growth chart for height and weight. The heavy line in the middle indicates the average gain over the years.

Male growth chart for height and weight. The heavy line in the middle indicates the average gain from two to twenty years.

People with sickle cell disease often worry about their weight and it is common to lose weight during a crisis and for it to be difficult to regain the lost weight when you get better. Checking your weight with your doctor or nurse at each outpatient visit allows you to track any changes together.

The heart rate or pulse is traditionally measured at the wrist but these days more often by a machine. Individuals with sickle cell disease often have a “resting tachycardia” which means a faster pulse rate than expected. The usual heart rate is about 60-70 beats a minute but someone with sickle cell disease may have a heart rate of 90-100 beats per minute. This is normal. It is because the heart has to work harder pumping the blood around the body to compensate for the anaemia. The more anaemic you are the less oxygen can be carried by the blood and the harder your heart has to work to deliver the same amount of oxygen to the tissues and muscles of the body as someone with a normal haemoglobin level.

With each heart beat a wave of pressure is generated which is what forces the blood around the circulation. The blood pressure measures that pressure wave. There are two figures in the blood pressure, the top figure is the peak pressure (systolic) and the bottom figure is the lowest pressure (diastolic). The normal blood pressure is 120/80 mm Hg (millimetres of mercury), people with sickle cell disease usually have slightly lower blood pressures than this and so recognizing high blood pressure, which is a very common disorder, can be difficult. Make sure you ask your doctor what your blood pressure is but remember that if you are unwell, in pain or anxious when the measurements are taken it might be higher than expected.

The respiratory rate is the number of breaths you take each minute, this is not so important when you are well but when you are ill it can be very important. The normal respiratory rate is about 16-18 breaths/minute. A chest crisis is often accompanied by an increase in the respiratory rate, or tachypnoea, when the respiratory rate might increase to 25-30 breaths per minute. Strong, opiate pain-killers tend to lower the respiratory rate. If the respiratory rate drops below 10 per minute then the strong pain-killers may have to be stopped or even reversed. Counting the respiratory rate accurately can be a very helpful guide to your doctors when you are unwell.

The oxygen saturation assesses the amount of oxygen that the blood is carrying. It is measured by a probe, which goes on the end of your finger and is expressed as a percentage.

A pulse oximeter to measure the oxygen saturation by passing a beam of light through a thin part of the body and assessing light absorbance. It will not be accurate if your nails have nail varnish or polish on them!

A pulse oximeter measures the oxygen saturation by passing a beam of light through a thin part of the body, in this case the finger tip, and assessing light absorbance. It will not be accurate if your nails have nail varnish or polish on them!

The doctors will want to make sure that your oxygen saturation is more than 95% at all times because a low oxygen level will only encourage more and more red blood cells to sickle. If it is low it can usually be easily corrected by having extra oxygen to breathe by a face mask or nasal cannulae. You should know what your oxygen saturation is normally when you are well, if it is less than 95% then it might indicate that the sickle cell disease has affected your lungs, which are having difficulty taking in enough oxygen from the air.

A urine sample is the last thing usually tested in outpatients by dipping a testing strip into the urine sample – what would this tell anyone?

The little squares on the testing strip are sensitive to a variety of chemicals in urine, the colour reaction of the squares gives an indication of the concentration of these chemicals. Whoever checks the results will compare the colours on the testing strip with the standard colours on the outside of the storage bottle to derive an approximate concentration.

Checking the urine dipstick against the colour coded results on the bottle.

Checking the urine dipstick against the colour coded results on the bottle.

There are two important reasons for testing the urine, firstly to see whether someone has a urine infection. These infections are common, especially in women, and do not always give rise to any symptoms. If an infection is present the testing strip will give a positive reaction for nitrites and leucocytes. Generally, if this is the case the urine sample will be sent off to the laboratory for the infection to be confirmed and for the bacteria causing the problem to be identified. The laboratory will also tell your doctor which antibiotics the bacteria is sensitive to. It is important to detect an infection and treat it with a short course of antibiotics because, untreated the infection may provoke a sickle cell crisis.

The other tests done on the urine sample are to detect any protein or albumin, sometimes called micro-albumin, or any haemoglobin in the urine. The presence of either of these in the urine may indicate that the sickle cell disease has affected the kidneys, which normally produce the urine, making them less effective as filters, so that some large molecules, such as albumin and haemoglobin, pass through instead of being retained. Minor damage to the kidneys with haemoglobinuria (=haemoglobin in the urine) and proteinuria (= protein in the urine) is common in sickle cell disease and does not mean that you have kidney failure. However, it is important to know about this and make sure that it is monitored and checked each time you go to outpatients. If the testing strip indicates that there is a significant amount of protein present your doctor will probably want to have this measured accurately in the laboratory.

People with sickle cell disease will have many, many blood tests throughout their lives. Although there are numerous things which can be measured in the blood most of the tests which are undertaken fall into a small group and are usually done to either check whether  someone is more anaemic than usual, search for signs of infection or dehydration or monitor the course of a painful crisis. Because people with sickle cell disease have so many blood tests and intravenous drips during their lives their veins are often very damaged and taking a blood sample can sometimes be very difficult. Always ask for an expert to take your blood, do not let any one practice on you – your veins are very precious! The most common blood tests your doctor will order include the following:

A blood count looks at the cells in the blood. Among other things the machines in the laboratory measure the number of red cells and the haemoglobin concentration. Almost everyone with sickle cell disease is anaemic, that’s why the condition is sometimes called “sickle cell anaemia”,. The usual haemoglobin concentration in a person with sickle cell disease is 70-80g/l (grams per litre), this is about half the haemoglobin concentration of a person with normal blood. The reason for this anaemia is that red cells containing sickle haemoglobin have a shortened lifespan, instead of living for the normal 120 days they may only survive for 10-20 days in the blood stream. In response to this the bone marrow works much, much harder producing huge numbers of red cells every day, but cannot fully compensate for the increased rate with which they are removed from the blood, hence the long term anaemia. You should always try and remember your own “steady-state” haemoglobin concentration, it will often fall further, maybe as low as 50-60g/l, when you are ill, recovering back to your normal as you get better.

The two other main blood tests are renal, or kidney, function tests and liver function tests.

Kidney function tests, sometimes called U&E’s, tell your doctor how well your kidneys are working. The kidneys keep the internal environment of the body stable by removing waste products of metabolism and maintaining the levels of salts and water constant. If there has been some sickle cell damage then the concentration of creatinine or urea in the blood will give an indication about whether this is getting any worse or not; as kidney function declines the concentration of urea and creatinine in the blood rises. If your kidneys are normally unaffected but the level of these markers is found to be high when you are ill this might indicate that you are dehydrated, which must be corrected either by drinking more or by intravenous fluids. Dehydration, or a lack or water in the body, like a low oxygen level, will only make the sickle cell crisis worse by promoting more red cell sickling. 

Liver function tests or LFT’s for short assess how well the liver is working but also, like the haemoglobin level, give an indication about how quickly red blood cells are being broken down. The liver works with the kidneys to maintain stable internal environment but is also a storage site for glucose and fat, which can be released for energy when required and it is also responsible for making, or synthesising, many important chemicals. One of the most important liver function tests is the concentration of bilirubin. This is a yellow pigment, formed from the breakdown of red cells and haemoglobin which is normally excreted by the liver as bile, and flows into the bowel via the gall bladder.

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This diagram show the process by which haemoglobin from broken down red cells is converted to bilirubin and excreted from the body. Starting at the top, the damaged, or senescent, red cells are broken down, or haemolysed, by macrophages, or mononuclear phagocytic cells. The haemoglobin is initially converted to heme, then to biliverdin (1) and finally to bilirubin and bound to a carrier protein, albumin, in the blood (2). Bilirubin is taken up by the liver and biochemically modified, or conjugated, in the hepatocyte, or liver cells (3 & 4). It is then excreted by the liver cells into the bile ducts and stored in the gall bladder (the small sack just underneath the liver), before being excreted into the small bowel, or duodenum, to aid digestion (5). Waste bilirubin is eventually lost in the stools.

Because people with sickle cell disease break down their red cells faster than normal they produce much more bilirubin, and the level of bilirubin therefore increases in the blood; the normal bilirubin concentration is about 15umol/l (micro moles per litre) whereas if you have sickle cell disease it is usually 70-80umol/l. Because bilirubin is yellow in colour it can be seen as a yellow discolouration of the whites of the eye. This finding is known as jaundice, which is often a sign of liver disease, but in the case of some one with sickle cell disease is only a reflection of increased red cell breakdown and should not be taken to imply any problem with the liver. You should try and remember what your normal bilirubin level is – when you are ill with a crisis it will always go up, sometimes to several hundreds but will again return to normal on recovery. The other liver function tests are collectively called “liver enzymes” or “transaminases”. Different ones are measured in different hospital laboratories but they include, aspartate transaminase (AST), alkaline phosphatase (alk. phos.), alanine transaminase (ALT) and lactate dehydrogenase (LDH). Many of them, like the bilirubin concentration, give the doctors information about how well the liver is working as well as how fast the red cells are being broken down.

A chest x-ray (CXR) is the most commonly ordered x-ray investigation because good oxygenation is so important if you have sickle cell disease. A CXR will give your doctor information about how well your lungs are expanding, whether there is any infection or pneumonia present in the lungs and whether a painful crisis is complicated by the acute chest syndrome.

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This is a normal CXR. The lung tissue is black because the lungs contain mainly air which transmits the radiation through to the x-ray film. The curved white shadows crossing the black lung fields are the ribs – 12 on each side. The collar bones or clavicles are the two straight bones at the top of the lungs with the dark shadow of the windpipe, or trachea, in between them. The shadow of the heart is in the middle, with the arch of the aorta, the main artery leaving the heart and taking blood to the body at the top left. The fine, white lines radiating out into the lungs on each side are blood vessels taking blood into the lungs to collect oxygen from the air. The lower limit of the lungs is marked by a large muscle, the diaphragm, which helps to expand and contract the lungs during breathing. beneath the diaphragm on the right is the liver.

The CXR also gives some information about the size and function of the heart. Many people with sickle cell disease will be told that their heart is bigger than normal, do not be alarmed, this is normal. Remember that that because of the long term anaemia your heart will beat faster than normal to make sure that enough oxygen is delivered to the muscles and tissues. The heart is a muscular pump and, just like any other muscle, the harder it works the bigger it gets. So, a large heart just means that your heart is working efficiently.

A CXR involves exposure to small amounts of ionising radiation and people sometimes worry about whether this is safe. Radiation exposure is measured in milliSieverts (mSv’s); we are all exposed to natural, background radiation all the time and the average annual exposure is 2.2mSv. A single CXR involves an additional exposure of 0.02mSv whereas a CT scan involves much higher levels, say 10.0mSv. A single CXR  therefore involves very little additional risk but, people with sickle cell disease may have many x-rays over a year and all of these exposures will add up. Always ask your doctor why an x-ray or scan is necessary; they should only ever be ordered if they will make a real difference to how you are treated or how quickly you will get better.

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About rogerjamos

I am a consultant haematologist who has worked in Hackney, London, UK with patients who have sickle cell disease for many years. Knowledge is power; the hope is that this blog will empower patients by putting them in touch with contemporary research into sickle cell disease and facilitating informed discussion on the issues raised. Dr Roger Amos MA, MD, FRCPath
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