Saturday, 2 August 2014

Investigations, Investigations, Investigations (Part 2)

Image courtesy of [anankkml] / FreeDigitalPhotos.net

The cornerstone of medicine remains the classic history and examination- let the patient tell their story, and the physician's hand of assessment can help pinpoint the diagnosis.

However, when diagnoses are unclear or can't be differentiated, that's where investigations come in. Sadly, today they're often over-used, but when used appropriately they offer a wealth of information that can help tailor all important management and get the patient on the path to recovery faster.

Following on from part one, where we looked at urine, this multi-part post will take a look at some of the common investigations performed, what they can tell us and the reasons why and when they should be used. Part two deals with perhaps one of the most important investigations available to modern medicine- blood!

Perhaps one of the most loathed tests for patients, as it involves needles, blood is an amazingly versatile substance, and it can tell us a lot about a patient.

PART 2- blood



Today we have thousands of blood tests that can check for almost anything in terms of pathology, from inflammatory markers to vitamin and drug levels, to autoimmune conditions, kidney and liver function, and even whether you've had a heart attack or not.

It would be impossible for me to go through every single one, so I'm going to stick to the routine tests performed.

Note that I've not included reference ranges here, as many hospital use different machines which have different units, and it can be confusing.

Depending on what it's used for, blood needs to be taken in various different coloured bottles, as different reagents are needed to identify the compounds.

Full Blood Count (Complete Blood Count)



This test assesses various blood components that come from the bone marrow. This includes haemoglobin (Hb), the molecule that carries oxygen in the blood; white cells (WCC, white cell count), the fighting force that give immunity to infection; platelets, the tiny fragments that stop bleeding, and lots of other smaller parameters that can all be useful.

Hb checks for anaemia (too little) or polycythaemia (too much), and in conjunction with other parameters (like red cell volume, mean cell volume) can also point to the cause (e.g. microcytic anaemia could be due to iron deficiency).

The WCC, if low, can indicate immunosuppression or bone marrow failure, or if high, acute infection or inflammation, or if very very high, leukaemia.

Finally, the platelets can give an indication of clotting ability and bleeding or thrombotic risk (e.g. low platelets, thrombocytopenia, can increase risk of bleeding, but too high platelets, thrombocythaemia, increase risk of clots in the body, including the coronary vessels of the heart, leading to a heart attck)

All in all, the FBC/CBC an extremely useful test for many different pathologies, and hence why it's included in the routine blood tests.

Biochemistry



This is a broad term that includes:

Urea and Electrolytes, looking at how the kidneys function, the concentration of important blood salts such as sodium and potassium, and how much kidney reserve you have.

- Liver function, looking at various liver enzymes, and how the liver functions. Specific liver enzyme increases can even tell us what's causing the problem, such as gallstones, alcoholic liver disease, or hepatitis. And even if the diagnosis is unclear, we can then use other tests like ultrasound to guide us.

- CRP (C-reactive protein). This is a non-specific marker of infection or inflammation. As such, when it's raised, it's not that helpful at all as any number of things can cause this, but when it's normal, it can help rule out serious infections, and if you repeat the test, we can also see if treatment is working (it should decrease). Some hospitals don't bother with it though, as it's so unspecific.

-Albumin and protein. Again these also act as markers of liver and renal function (if low, it means the liver's manufacturing capability is poor due to damage, or it could mean the kidneys have lost their filtering ability and all the protein is being excreted into the urine). We can also identify if there are any nutrition or absorption problems from the gut. If abnormally high, this can also be a sign of multiple myeloma. However, like with CRP, it's not a very specific test and so it needs to be read in the context of other results and the patient's symptoms.

Lactate. This is a marker of anaerobic respiration in the body. Why is this important? Well, our cells need oxygen to survive, and normally use oxygen in aerobic respiration to create energy. But in oxygen starved states (such as running a marathon, or if you've got a severe chest infection), the body is so low in oxygen it needs to convert to a non-oxygen means of making energy. It's not good to have lactate in high concentrations for a prolonged period, and it's a good marker to look at the severity of an acute illness where the body is severely starved of oxygen (such as chest infections, sepsis, major heart attack etc).

- Hormone profiles include the thyroid hormones (thyroid stimulating hormone, TSH, T4 and T3), the female and male sex hormones such as oestrogen, progesterone, testosterone, FSH [follicle stimulating hormone] and LH (luteinising horming], prolactin [important in breast feeding and benign tumours of the pituitary], cortisol [natural steroid]). These are important for endocrine problems.

-Glucose and HbA1c are useful for diagnosing and monitoring diabetes. HbA1c is 'glycosylated haemoglobin'. The premise of this test is to look at long term diabetic control. It works because it's measuring the amount of glucose that has stuck to part of the haemaglobin we talked about earlier. As the life span of a red cell (where the haemaglobin is) is about 90 days, we can therefore see that the higher the amount of glucose stuck to the cell, the higher the blood sugar has been over those 90 days. Targeting the HbA1c can be useful to prevent or reduce the risk of diabetic complications.

-Drug levels. Digoxin, lithium, sodium valproate, pheyntoin, aspirin, paracetamol (acitomenophen) and many other drugs can also be measured, either in terms of acute poisoning or overdose, or to monitor safe dosing and compliance to treatment.

Cardiac Troponins. These tiny protein molecules are release when the heart muscle (myocardium) is damaged, and can be a great indicator if there has been heart muscle damage, either from a heart attack, atrial fibrillation or infection. While they are pretty specific for this, note that some conditions like chronic renal failure, pulmonary embolism and others can cause a 'false' high level, so interpret with caution.

Clotting



This looks at the clotting factors produced by the liver (which work in conjunction with platelets to stop bleeding). If these are abnormal, it can be an indicator of liver disease or severe infection.

The INR (international normalised ratio) is also used as a standardized marker for warfarin therapy, which is used in the treatment of blood clots (thromboembolism) and in prevention of formation of clots.

The APTT (activated partial thromboplastin time) is used to monitor heparin therapy.

Like with liver function, patterns of clotting abnormality can pinpoint the diagnosis. We can also test for some specific clotting factors (like factor 8, which is affected in haemophilia), which can help exclude or confirm diagnosis.

Group and Save/ Cross Match



This is used for patients requiring or who may require a blood transfusion, either as an emergency (cross match, the sample is processed very quickly but it's only basic type specific blood) or after an elective operation (group and save, processed more slowly but more type specific blood is issued). This test must be repeated after a transfusion is given, as a patient may develop antibodies and so different blood needs to be matched to prevent an immune reaction.

Immunology Testing



These highly expensive and non-routine tests look for antibodies, typically those causing autoimmune conditions like rhematoid arthritis These tend to be quite specialised and need to be interpreted in a clinical context. Alone they are quite useless, as the results are given in titres, not absolutes, and have a wide interpretation.

Examples include rheumatoid factor (RF), although you CAN have RA with a negative RF), anti-neutrophil antibody (ANA), anti-double strand DNA (specific for systemic lupus erythematosis, SLE) anti-Ro, anti-La (it's like a piano song isn't it?), and many more.

- Serology. This looks at antibodies against infection, such as rubella, chicken pox, and others. More complex testing is done for things like HIV and Hepatitis B. This helps us to see if a patient has had a previous infection to this disease, is having a current infection, and if they have immunity. This is also used for to check that a vaccine (typically Hepatitis B) has worked.

Whew, that was a lot of blood tests, and there's still so many more I haven't covered!

Part 3 will move onto another very common set of testing- images.


Further reading:

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