Performance of screening for biochemical parameters and repeated assessment of these is a widespread procedure in elite sports medicine and exercise physiology. Several recent studies have assessed the clinical utility of screening for iron-related parameters in elite athletes, but to date there have been no studies on the clinical utility of screening for other biochemical parameters.

A number of authors have suggested that widespread screening should be performed in an effort to produce a set of normal ranges for elite athletes, and these reference intervals are becoming available.1^–4 Others have suggested that screening of some biochemical variables when athletes are in the rested state may be useful as a baseline for biochemical monitoring of training,5 and it has also been suggested that such monitoring may be useful in the prediction of the onset of overtraining.6 Each of these are worthy goals, but as biochemical monitoring is relatively expensive, and involves some discomfort and inconvenience, it is important that each of these potential applications is shown to be worthwhile and that any clinical application of biochemical screening be determined. The aim of this prospective clinical case series was to assess the clinical application. The uniqueness of this study lies in the fact that biochemical testing was performed in association with clinical consultations and that each abnormality found was followed up with both a clinical consultation and repeat testing.

METHODS

This study was approved by the ethics committee of the Australian Institute of Sport. All procedures conformed to the National Health and Medical Research Council guidelines for experimentation with human subjects, and all participants gave their informed written consent before participation.

All 100 elite athletes (56 males and 44 females; mean age for both groups 19 years (range 16 to 27)) presenting to the Sports Science/Sports Medicine Centre at the Australian Institute of Sport for routine medical screening during a period of approximately 1 year were entered into the study. All of the athletes were in training and involved in some form of training once or twice a day for at least 6 days of each week. The participants underwent a standard medical history and examination by a sports physician, after which blood (12 ml) was drawn under aseptic technique from a vein in the cubital fossa. Blood was drawn immediately after the athlete lay in a recumbent position and was transported immediately to an adjacent haematology laboratory where analysis was performed on the same day.

Usually, because of training demands, the timing of blood collection was not standardised in relation to periods of training and time of day. Therefore parameters potentially affected by training were almost certainly perturbed from true baseline (resting) levels, but as elite athletes do not generally stop training because of routine blood tests, the results represent a real rather than an artificial situation. We were not able to correct for diurnal variation, which may occur for serum iron, for example, but this did not appear to be clinically relevant in any of the cases. All athletes whose screening blood tests revealed abnormalities were interviewed for clinical symptoms, and repeat blood testing was performed on all but 16 athletes; the exceptions were 9 who had iron depletion, six of whom, after discussion of the results, declined further testing and one moved away from the area. The nine athletes with iron depletion were, after dietary assessment, started on iron supplements to be followed up 3 months later at the conclusion of their supplementation programme.

The following biochemical parameters were measured on an Hitachi 911 analyser (Roche Diagnostics, Indianapolis, USA) using Roche reagents: serum iron, ferritin, transferrin, percentage transferrin saturation, sodium, potassium, chloride, calcium, magnesium, phosphate, urate, urea and creatinine, total protein, albumin, creatine kinase (CK), lactate dehydrogenase (LDH), aspartate aminotransanimase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), gamma-glutamyl transpeptidase (GGT), total bilirubin, cholesterol and triglycerides (non-fasting), and random glucose (blood sugar levels; BSL)

A full blood count was also performed and, although the results are not the subject of this paper, these were sometimes used to assist in further assessment of the biochemical abnormalities detected.

Reference intervals were those of the haematology and biochemistry laboratory at the Australian Institute of Sport. The intervals were derived from several thousand elite athletes tested over a period of >5 years.

RESULTS

The athletes were recruited from 11 sports (table 1). No abnormalities on biochemical screening tests were found for 18 athletes (10 male, 8 female). In total, 194 abnormal results were found in 82 athletes; 115 abnormalities in 46 male and 79 in 36 female athletes.

Abnormalities noted more than once are shown in table 2.

In 43 individual tests, the results did not return to normal on repeat testing (table 3).

In 11 cases the abnormalities were consistent with clinical entities. Three cases had confirmed hypercholesterolaemia. Four cases in which iron-related parameters were abnormal were explained by concurrent infections. One case of increased serum iron was associated with ingestion of iron supplements, and one case of increased GGT was associated with alcohol ingestion. One athlete with increases in LDH, AST, ALT and ALP had, on follow-up, associated lymphocytosis and positive serology for Epstein–Barr virus infection; this person was asymptomatic. One athlete with a persistent rise in transferrin saturation and normal serum ferritin was diagnosed with haemochromatosis after testing homozygous for the C282Y mutation.

The 9 athletes with serum ferritin <30 ng/ml were deemed to be at risk of iron depletion and were placed on supplementation without retesting.

Seven athletes who were not iron depleted did not have follow-up blood tests. Four athletes decided not to have further tests when it was suggested that the probable cause was a normal response to training. Two athletes had a GGT level of 10 U/l (normal range 11 to 49 U/l) in the presence of normal other liver function tests and one athlete moved away from the area.

DISCUSSION

Screening may be defined as an attempt to detect disease in asymptomatic people. For screening to be effective, three criteria must be taken into consideration: the characteristics of the target population, the diseases targeted for detection and the nature of the proposed tests. Tabas and Vanek7 have suggested that, in relation to the population concerned, the main considerations are: the prevalence of the disease, the accessibility of the population and the likely compliance with follow-up diagnostic tests and treatment. In relation to the disease, detection should have a significant effect on the quality or length of life, acceptable methods of treatment should be available and the early detection should improve outcomes from treatment in the asymptomatic phase. The tests should be sensitive enough to detect the disease in the asymptomatic period, sufficiently specific to provide acceptable predictive value and be acceptable to patients.

In many cases, few of these factors are considered before routine biochemical screening is performed on elite athletes, and indeed much of the screening performed appears to be for reasons other than detection of disease in the asymptomatic athlete.

The clinical utility of biochemical screening using multiple parameters has often been assessed in the general non-athletic population. Based on a review of seven studies, Cebul and Beck8 concluded that biochemical profiles are not warranted in asymptomatic people and that determination of serum glucose, cholesterol and “with modest enthusiasm”, serum creatinine, is sufficient. In a more recent study, Ruttiman9 et al reported on screening using 23 biochemical parameters in 493 patients (mean age 41 years). Sixty new diagnoses were made, resulting in new management in 5% of patients. Only 0.2% of tests led to new management. The four most common new diagnoses were hypercholesterolaemia (52%), hypertriglyceridaemia (13%), hyperuricaemia (10%) and “slight abnormalities of liver enzymes of unknown origin” (7%). The rate of individuals with abnormal routine tests was 88%. These authors suggested that most of the clinically significant abnormalities could have been detected by use of a short test panel comprised of serum cholesterol, glucose and ALT. Based on these studies, it could quite reasonably be suspected that the clinical utility of biochemical screening in a very fit and generally healthy population such as that composed exclusively of elite athletes might be somewhat limited.

In the present study, the most common abnormality (27 cases) was an increase in AST (range 38 to 238 U/l, 18 cases <50 U/l). Twelve of these were associated with CK levels >780 U/l (the upper limit of normal for the Australian Institute of Sport laboratory), suggesting a skeletal muscle origin; 21 were associated with other liver function tests that were completely normal; and 6 with increases in ALT. At both the initial and follow-up consultations, there were no symptoms or signs to suggest liver disease or excessive alcohol ingestion. Results for all except six of these athletes returned to normal on repeat testing, which was not performed during the resting state. Based on data from a study of 579 club-level athletes, the upper limit (mean plus 2SD) of normal might be as high as 55 U/l, which would reduce the number of abnormal results for this parameter to eight.10

Noakes,11 as long ago as 1987, described earlier papers that indicated that a rise in AST can occur following activities as varied as walking on a treadmill for 5 minutes, a boxing competition, swimming, rowing and callisthenics. In military training, up to sixfold increases in AST have been found,12 and very large increases have been reported after ultramarathon running.13 It would appear therefore that increases in AST in elite athletes in training, particularly in association with increases in CK, normal results on other liver function tests and no other clinical symptoms and signs, are of no clinical significance. As ALT has also been shown to increase in relation to exercise, an increase in this enzyme associated with a mild increase in AST is almost certainly of little significance.

Several commonly used medications including some antibiotics, 3-hydroxy-3-methylglutaryl coenzyme A (HMG Co-A) reductase inhibitors, drugs to treat epilepsy and herbal preparations can cause rises in aminotransferases. In the context of athletics, the use of non-steroidal antiinflammatory drugs should be particularly considered.14 None of the athletes were using any of these medications at the time of screening. The oral contraceptive pill may occasionally have a similar effect and is also highly relevant in this population.15

An increase in serum bilirubin was found in 11 male and one female athlete (range 20 to 52 μmol/l). Of this group, eight cases were associated with normal results for other liver function tests. On review, bilirubin was still high in nine cases. Seven of these were associated with normal liver function tests. Of the others, one had increased AST and increased CK. The other had an increase in AST and normal CK, and had been previously investigated and been diagnosed with Gilbert syndrome. Although it was not proven that the bilirubin in these cases was unconjugated, the absence of abnormalities in the other liver function tests and the normal full blood counts in all cases suggests that these athletes also have Gilbert syndrome. The male preponderance is consistent with previous studies, but is greater than is generally reported and the incidence of 9% is also higher than usual. Despite the fact that endurance exercise has been shown not to lead to an increase in bilirubin in athletes with Gilbert syndrome,16 an increase in this parameter has been found after a number of forms of exercise17 and so perhaps it is not surprising that more cases of this condition are identified in those who are habitually active than in the general population.

A rise in serum phosphate (range 1.50 to 1.86 mmol) was found in 13 cases, none of which was associated with hypocalcaemia. Nine were found to be normal on repeat testing, and of the four who did not, the range was 1.47 to 1.60 mmol/l (normal 0.87 to 1.45). The response of serum phosphate to exercise is quite variable across different forms of activity. For instance, Clarkson et al18 found no change in this parameter after 50 maximum eccentric contractions of the elbow flexors, which induced marked changes in CK, whereas Nagel et al found a 73% increase in runners competing in a 24-hr foot race.19 There are no reports of negative clinical consequences of changes in serum phosphate in athletes.

A rise in serum urea was found in 12 athletes in our study, none of which were associated with increases in serum creatinine, and urinalysis was normal in each subject. Only two athletes did not have normal results on repeat testing and in both cases, the second result was close to the upper limit of normal. Increases in this parameter after exercise are very well described.18 19 They are thought to reflect increases in production secondary to degradation of amino acids after muscle cell damage. Other factors relevant to sport, such as increases in ingestion of amino acids and dehydration, can also lead to increases in serum urea. The type of change found in our study, particularly if associated with normal serum creatinine, is of no clinical significance in athletes.

Nine athletes had a rise in serum urate, and all had returned to normal on repeat testing. Mean values at rest in elite athletes have previously been assessed, with significant reductions found when both cyclists and skiers were compared with controls.3 In relation to exercise, changes in this parameter are similar to those of urea, with the source of the metabolite again being related to breakdown of amino acids.18 19

Abnormalities in blood glucose were found in 11 male (decreases) and 5 female athletes (increases). Mean values at rest in elite male athletes have previously been assessed, with significant reductions found when both cyclists and skiers were compared with controls.3 In our study, four athletes with decreased blood glucoseand two with increased blood glucose did not have normal results upon repeat testing. Neither the initial tests nor the follow-ups were associated with symptoms, and do not seem be of clinical significance

The three cases of hypercholesterolaemia that were found represent three of the four test results that were of clinical significance. In one athlete, the family history was positive for hyperlipidaemia. Another, in which the repeat level was 5.32 mmol/l (normal range 1.3 to 5.2) will most likely be controlled through dietary intervention. The other two athlestes were started on HMG-CoA reductase inhibitors, which resulted in significant reductions in serum cholesterol. The 3% incidence of hypercholesterolaemia, defined as total serum cholesterol >5.2 mmol/l, is markedly different to the 20% incidence in professional skiers and the 30.7% incidence in professional cyclists previously reported.4 The discrepancy may be explained by the differences in mean age of the athletes in the studies. It may be prudent that if screening is to be performed, assessment of serum cholesterol in older athletes should be included.

One case of haemochromatosis was found in our study. This athlete initially had increased serum iron and transferrin saturation (69%) and a persistent increase in transferrin saturation (57%) on repeat testing. Serum ferritin was normal on each occasion (103.4 and 88.8 ng/ml), respectively. The athlete was found to be homozygous for the C282Y mutation.

In terms of individual athletes, only 4% of biochemical screening tests had a clinically significant effect. In terms of individual tests, only 4 of 2500 (0.16%) tests were of clinical significance. These findings and the predominance of hypercholesterolaemia are remarkably similar to those of Ruttimann et al in their study of patients in general.9

In general medicine, screening of healthy young people for abnormalities of any of the parameters that were the subject of this study, including both serum cholesterol and blood glucose, is not currently recommended.20 It may be, however, reasonable to screen those with a positive family history for early vascular disease, hyperlipidaemia or haemochromatosis.

General screening of elite athletes for a wide range of biochemical parameters is not justified on clinical grounds. It is likely to reveal only changes that are well documented as being a result of training or false-positive readings that are close to the reference limits of the normal population and are related to the statistical methods used for definition of a “normal” population. Such screening is further complicated by the fact that true resting values are very difficult to assess in athletes who are training every day, if not twice a day, and important considerations such as normal biological21 and diurnal variability,22 hydration, nutritional and fasting status are difficult to control.

In this study, 82 athletes were recalled for repeat testing. Repeat testing can engender significant concern in athletes, who may suspect that they have a serious abnormality and also leads to what now appears be to unnecessary discomfort and expense.

As the evidence for biochemical monitoring of training using the parameters included in this study is sparse, and in the absence of proven biochemical markers of overtraining, it would appear that the only reasonable use for biochemical screening of elite athletes is in studies that seek to determine reference intervals for specific athletic populations. All other biochemical screening of athletes, with the exception of that for serum ferritin, the utility of which has been previously shown, should be re-evaluated.