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Lupus can, in the vast majority of patients, be diagnosed quite precisely with blood tests. The hallmark of active SLE is the finding in the blood of a variety of antibodies - a group of proteins - which have the property of reacting against various chemicals in the nucleus of the cell.
- Anti-nuclear anti-bodies (ANA)
- Anti DNA Anti-bodies
- Blood Count
- Kidney Function
- Liver Function
- Other tests
These so-called anti-nuclear antibodies can, as a group, be measured simply and inexpensively by adding a drop of the patients' serum onto a cell nucleus, - the "anti-nuclear antibody" or ANA test (often called the fluorescent ANA or FANA test in America).
In practice, a drop of serum is placed onto a carefully prepared nucleus (the source of the nuclei varies from lab to lab). Anti-nuclear antibody is then detected by adding a special marker protein containing a fluorescent "label". The nucleus with attached ANA is seen to fluoresce under the special fluorescent microscope.
The test is positive in almost 99% of SLE patients, though it may be negative in patients with discoid lupus. Thus it is useful as a "screening" test. Unfortuantely, from the physicians point of view, it is not absolutely specific for SLE, and positive tests are occasionally seen in other, non-lupus, conditions such as Rheumatoid arthritis.
The LE cell (discovered in 1948) is simply a more old-fashioned and less sensitive method of looking for ANA. It is also time consuming and has been largely abandoned. The ANA test is inexpensive, simple, and available in most major hospitals and laboratories.
In the late 1960's it was discovered that the serum of lupus patients contained antibodies reacting against DNA (desoxy-ribose-nucleic acid). This finding led to the development of tests for this, the most specific of antibodies in the diagnosis of SLE. Using sensitive assays in the laboratory, it is possible to obtain a precise estimate of the concentration (or titre) of those antibodies. The most widely used of these tests is the DNA-binding test, which gives the clinician a broad guide to changes in disease activity, and which is now used worldwide as a barometer of lupus activity.
For years, workers have attempted to devise methods of reliably measuring complexes of antibody and antigens such as DNA in the bloodstream, but as yet no single test is perfect. Some complexes act like blotting paper and soak up one of the constituents of the blood - so called "complement". Thus the finding of a low "complement" level in the blood is indirect evidence that potentially harmful complexes are circulating.
Anaemia is common in lupus and may be the result of a variety of factors. Occasionally, the number of platelets falls, leading to "purpura" - small red pin-point spots of bleeding in the skin.
The ESR (erythrocyte sedimentation rate) is a time honoured test used throughout the field of medicine as a "general" guide to disease activity. It is not specific for lupus, but, as in the case of a child's temperature, gives a general guide to the presence of inflammation. In active lupus, the ESR (normally under 20) may rise well above 100.
Possibly the most important test in lupus is analysis of the urine. The most common (and usually earliest) sign of any inflammation of the kidney is the demonstration of protein in the urine. This can be detected by dip-stick testing, but for more precise quantitation, a 24 hour sample is preferable. The other important urine test is by examination under the microscope, in order to detect cells - again a guide to inflammation.
If there has been any impairment of kidney filtering, certain "waste" nitrogen-containing chemicals (eg urea and creatinine) damn up in the blood stream. Their blood level provides important monitoring help.
The liver is rarely affected in lupus, though other factors such as drugs or virus infections can cause transient changes in liver function.
The other tests ordered by the physician will depend to a certain extent on the pattern of the lupus. These tests may include X-rays (especially chest X-rays), breathing tests, body scan, MRI, etc, depending on the organs involved.
The tests can be divided into those specific for lupus, such as the anti-DNA (or "DNA-binding") test, and those which mirror more general disease changes, such as the blood count. Which tests are performed depends largely on the type of disease the patient has, though it has been our practice for many years that when we are referred a new patient with lupus, we perform a "streamline" investigation by doing all the available blood tests on the initial sample, as well as ordering a routine "protocol" of investigation. In this way, a reasonably precise assessment of the extent of the disease can be made at the outset.