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A few biological examinations are required for the precise diagnosis of cancer: for instance, measuring the β - HCG value for placental choriocarcinoma, β-HCG or α-fœto-protein for testicular or ovarian tumours, thyroglobulin in thyroid tumours or ACE for medullary thyroïd carcinoma.

Other simple biological examinations are useful for classifying tumours (research for liver metastases by hepatic enzymology) or for general status before therapeutic procedures (complete blood count, study of renal function, oxymetry).

Other examinations should be systematically performed in the case of an altered general status, hydration disorders or agitation (for example: screening for hypercalcaemia, hyperkaliema) - see emergency section.

Other blood tests are interesting for monitoring disease evolution (tumour markers - cf. specific chapter) or treatment toxicity (blood count during chemotherapy).

New prognostic factors are emerging from molecular biology and are modifying the standard check-ups for certain tumours.

However, blood sample miniaturisation and increasing reliability of biological devices should not obscure the fact that each biological procedure is an aggression for the patient, and, as such, should be limited as much as possible.

Patients expect a great deal from biology (as from other paraclinical examinations). Asking for a blood test implies offering a precise explanation on its results. The clinician should, first of all, ask himself if there is a useful therapy that could be applied before dosing tumour markers. For a patient, the knowledge of both the presence of a poor marker and the absence of any useful therapy is psychologically quite difficult to tolerate.

Furthermore, all of these more or less useful examinations are a supplementary cost for health providers.