Informed consent and confidentiality are required for all investigations, and the advantages should clearly outweigh any dangers or disadvantages.
Blood tests can help determine disease states, but should be requested only when clinically indicated. There is always a danger of needlestick injury. Furthermore, abnormal ‘blood results’ do not always mean disease and false-positive results are possible. Serum is used for assaying antibodies, which can help diagnose infections and autoimmune disorders, and for assaying most biochemical substances (e.g. ‘liver enzymes’).
Microbiological diagnosis is based on either demonstration of the microorganism or its components (antigens or nucleic acids) directly in samples or tissues, which are best used as results are speedily obtained. The demonstration in the serum of a specific antibody response can be helpful.
Salivary flow determination
Unless the baseline salivary flow rate for an individual patient is known, it is impossible to be certain if there has been a reduction in salivary flow, since the salivary flow rate varies widely from person to person. Salivary flow rates also vary over time and so estimates should be taken on several occasions.
Normal and reference values are shown in Table 7.1.