Clinical chemistry at the extremes of age

Chapter 21 Clinical chemistry at the extremes of age



Old age


The investigation and management of illness in the elderly poses a number of special problems, for both the physician and the clinical biochemist. These include:



There is no precise definition of what constitutes ‘elderly’: the changes in physiological function that occur in older people correlate only loosely with chronological age and can occur much earlier in individuals with conditions characterized by premature ageing (e.g. progeria). Nevertheless, many conditions are more common in the elderly than in younger adults; examples of such conditions of particular relevance to clinical biochemistry include diabetes mellitus (see Chapter 11), osteoporosis (see p. 260), Paget’s disease of bone (see p. 262) and thyroid diseases (see p. 166). Furthermore, the presentation of diseases in the elderly may be different from that usually seen in younger people. Thus myocardial infarction may present with confusion, consequent to a reduction in cerebral blood flow, rather than with chest pain; the presenting feature of diabetes mellitus may be one of its complications, for example ischaemic ulceration, rather than polyuria and thirst. Case histories 21.121.4 provide more detailed examples of these problems.


The function of some organs declines with age; although this may not be apparent in healthy individuals, the consequently decreased reserve capacity may become apparent in even mild disease. In addition, functional decline may be accelerated by even mild disease. For example, the glomerular filtration rate decreases with age and so does the creatinine clearance. However, plasma creatinine concentration changes little because creatinine production also falls with age; this reflects a decrease in muscle mass and often also in meat consumption. Despite the fall in the glomerular filtration rate, renal function remains sufficient for normal homoeostasis, although it may not be adequate to allow complete excretion of a drug or to sustain any further decrease in glomerular filtration without a failure of homoeostasis. Renal responsiveness to vasopressin, thirst sensation and the aldosterone response to renin all decrease with increasing age, putting elderly people at greater risk of disorders of fluid balance and composition.


Elderly people, particularly if they have impaired mobility or live alone, may have poor nutrition and are especially prone to vitamin D deficiency (see p. 328). Also, they are more likely than younger people to be taking medication, often multiple, which may have adverse effects in addition to the expected therapeutic effects.



Reference ranges


Such changes in normal function mean that the reference ranges applicable to healthy adults may not be applicable to the elderly, while the increased incidence of many diseases with increasing age makes it difficult to obtain data on healthy people. Ideally, laboratories should construct age-related reference ranges for age-dependent analytes (where appropriate: it is not, for example, for glucose) (Fig. 21.1), but in practice this is not always done.



This problem is exemplified by the enzyme alkaline phosphatase. Common causes of raised activity of this enzyme in the plasma in the elderly include malignant disease with metastasis to bone or liver, osteomalacia and Paget’s disease of bone. In the UK, the prevalence of Paget’s disease exceeds 5% in people aged over 60. Many cases are mild and clinically silent, being discovered only after a raised plasma alkaline phosphatase has been found, often as part of a biochemical screening test. Asymptomatic patients with Paget’s disease do not require treatment, but screening programmes are only worthwhile if abnormal results are followed up. How extensively this can be done may be governed by economic factors. The practice in many laboratories is to assume that, in the absence of any clinical or other laboratory evidence of disease, an alkaline phosphatase of up to one and a half times the upper limit of normal for young adults does not justify further investigation in an elderly subject. The plasma concentrations of urate and low density lipoprotein cholesterol also tend to rise to some extent with increasing age.




Screening


The higher prevalence of many diseases in the elderly provides some of the justification for screening programmes. If a condition has a high prevalence in a population, the predictive value of a positive test is much higher than if it is low (see Chapter 1). Such screening may be carried out in general practice, at over-60s clinics, in older people’s assessment centres or on admission to hospital. The biochemical investigations that should form part of such a screen (Fig. 21.2) reflect the diseases that are of particular concern in this age group, some of which have been mentioned above. Plasma potassium is included because diuretics are frequently prescribed in the elderly and, according to the type used, may cause hypokalaemia or hyperkalaemia. The possible influence of intercurrent disease or drugs (particularly lithium and amiodarone) on tests of thyroid status must be borne in mind. The results of such tests may erroneously suggest thyroid disease in a patient who is ill for some other reason (sick euthyroid syndrome) and it is best to avoid doing these tests at such a time. In the UK, it is recommended that all individuals engaging with healthcare should undergo a simple screen for malnutrition. This is particularly important in the elderly.






image Case history 21.4


An elderly woman was admitted to hospital after she had fallen at home and fractured her femur. She was a recluse and rarely went out, depending on a home help to do her shopping.


In addition to the fracture, a radiograph showed typical features of osteomalacia.


Apr 3, 2019 | Posted by in BIOCHEMISTRY | Comments Off on Clinical chemistry at the extremes of age
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