Biochemical investigation of rheumatic diseases

It is important for rheumatologists to have an understanding of biochemical tests including an awareness of their limitations. The biological variability of an analyte both within and between individuals, the limitations of the measurement technology, the sensitivity of laboratory internal quality control and external quality assurance procedures, as well as interlaboratory variations in practices including sample collection procedures, may all impact on the interpretation of a result. Biochemical tests are often requested to monitor organ-specific dysfunction arising as an adverse consequence of pharmacotherapy or as a component of a systemic rheumatic disease, although dysfunction may also reflect infection or coincidental pathology. Patients with rheumatic diseases are at high risk of renal and hepatic disease. Serum creatinine and its derivative estimated glomerular filtration rate (eGFR) are the most readily available surrogate markers of GFR and are used to assess renal impairment and monitor its course. However, the use of creatinine alone lacks sensitivity and a substantial loss of function must occur before creatinine levels are increased. Additional biochemical screening for kidney damage can be performed by assessment of glomerular integrity, including proteinuria or albuminuria and haematuria. A wide spectrum of rheumatic diseases can affect the liver with various degrees of involvement and hepatic pathology. These often present with cholestatic or hepatitic biochemical profiles. The medical management of rheumatic diseases also involves medications that are hepatotoxic, and routine monitoring of liver function is recommended. This approach is not problem-free and may be improved by quantitative determinations of non-invasive markers of liver fibrosis in the future. Together with imaging techniques, biochemical tests play an important role in the assessment and differential diagnosis of metabolic bone disease.