Laboratory Investigations

Radiologists should obtain a list of "normals" from their local clinical laboratory. There is a very considerable variation in the normal results of many tests carried out on tropical patients, not just due to unreliable laboratory technique, but because of population differences. Normal standards quoted in European or North American textbooks often will not apply. For example, the normal serum albumin and globulin levels may be equal. (There is evidence to suggest that black people have a genetically determined lower albumin and higher gamma globulin level than whites and that this may be further but only partly altered by environment.) The average white cell count in a local population may be higher or lower than in other countries and yet be "normal". Almost all biochemical and hematological investigations differ in this way. There are published figures for most localities, but they are too numerous to reprint here and, moreover, they can vary within 60 miles (100 kilometers) or less. Any radiological diagnosis that must be correlated with a laboratory or clinical test can only be valid if the local standard is known and accepted.
When a patient whose normal home is in the tropics comes to a different climate there is some delay in physiological adjustment. For example, the sodium chloride content of sweat takes some days to adjust, whether it be to a tropical climate or to a temperate climate, and if the patient is subjected to partial air conditioning in his or her environment, this adjustment takes even longer. Thus, depending on many factors and particularly economic levels, the delay in changing to the local "norm" may be significant or may not happen; this must be considered in assessing radiological findings. Remember also that other tests, such as the skin test for tuberculosis, may be positive and therefore not particularly significant, because in the patient's home country almost everyone will have a positive test. This applies to skin and serological tests for many parasites and the significance must be related to the usual background in the population. Unfortunately, the opposite may also happen; a positive lepromin test for leprosy may be found in those who have never been in contact with Mycobacterium leprae.