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Age and gender as risk factors for hyponatremia and hypernatremia
Clinica Chimica Acta 337 (2003) 169 – 172 www.elsevier.com/locate/clinchim
Age and gender as risk factors for hyponatremia and hypernatremia Robert C. Hawkins * Department of Pathology and Laboratory Medicine, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore Received 28 July 2003; received in revised form 8 August 2003; accepted 12 August 2003
Abstract Background: This study assesses gender and age as independent risk factors for hypo- and hypernatremia and describes the prevalence of hypo- and hypernatremia in different population groups. Methods: Details of all serum Na results with accompanying patient demographics for 2 years were downloaded from the laboratory database into Microsoft Access for multiple logistic regression analysis using SPSS. Female gender and age < 30 years were the reference groups. Results: Data from 303,577 samples on 120,137 patients were available for analysis. Prevalence at initial presentation to a health care provider of Na < 136, < 116, >145, and >165 mmol/l were for acute hospital care patients: 28.2%, 0.49%, 1.43%, and 0.06%; ambulatory hospital care: 21%, 0.17%, 0.53%, and 0.01%; community care: 7.2%, 0.03%, 0.72%, and < 0.01%. Age odds ratios rose with increasing age to 1.89 and 8.70 (Na < 136 and < 116 mmol/l) and 7.09 and 24.39 (Na>145 and >165 mmol/l, respectively) for age >81 years. Male gender was a mild risk factor for Na < 136 mmol/l and was otherwise unimportant. Conclusions: Hyponatremia is a common but generally mild condition while hypernatremia is uncommon. Increasing age is a strong independent risk factor for both hypo- and hypernatremia. Gender is not an important risk factor for disturbances of serum Na concentration. D 2003 Elsevier B.V. All rights reserved. Keywords: Age; Gender; Hypernatremia; Hyponatremia; Risk factor
1. Introduction There is little agreement on the prevalence of disturbances of sodium balance or on the importance of gender and age as markers of risk. Previous estimates of hyponatremia of 0.2 –29.8% [1,2] and hypernatremia of 0.3 – 8.9% [3,4] have varied with the differences in the definitions of the disturbance and the population surveyed. Most studies on hyponatremia have examined elderly and/or psychiatric populations with particular emphasis on psychotropic * Tel.: +65-6-3578943; fax: +65-6-2536507. E-mail address: [email protected] (R.C. Hawkins). 0009-8981/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cccn.2003.08.001
medications as a cause of hyponatremia [5,6]. Little has been published on the comparative prevalence of hypo- and hypernatremia in different patient populations (hospital or community). Female gender has been suggested as a risk factor for psychotropic [7], diuretic [8], and exercise-induced [9] hyponatremia. While some studies show increasing age to be a risk factor [7], others show no association with age [8,10]. This study assesses gender and age as independent risk factors for hypo- and hypernatremia in a mixed community/hospital population and describes the prevalence of different levels of dysnatremia (hyponatremia and hypernatremia) in different population groups.
170
R.C. Hawkins / Clinica Chimica Acta 337 (2003) 169–172
Table 1 Prevalence of hyponatremia and hypernatremia in different patient groups at presentation Acute Ambulatory Community Age>60 Age>70 hospital hospital care (%) years years care (%) care (%) (%) (%) Na < 136 mmol/l Na < 135 mmol/l Na < 126 mmol/l Na < 116 mmol/l Na >145 mmol/l Na >155 mmol/l Na >165 mmol/l
28.2
21
7.2
23.6
27.3
22.1
14.7
4.3
18.2
21.7
2.6
0.96
0.14
2.2
2.9
0.49
0.17
0.03
0.44
0.6
1.43
0.53
0.72
1.3
1.7
0.22
0.05
0.01
0.2
0.3
0.06
0.01
< 0.01
0.05
0.09
2. Materials and methods Tan Tock Seng Hospital Biochemistry Laboratory services a 1200-bed acute care hospital and its attendant outpatient services, and three community-based primary care polyclinics. All patients are admitted to the hospital via the emergency department. Serum Na measurements are performed on two Roche 917 clinical chemistry analysers using indirect ion-selective electrode technology. A 95% reference interval of 136 – 145 mmol/l was established in 1999 based on 560 normoglycemic multiphasic health screen samples. There were no significant differences in Na concentration or age between male and female reference individuals (mean age 47 years). Details of all serum Na requests for the last 2 years (2001 –2002) were downloaded from the laboratory information system into Microsoft Excel and then
Table 2 Odds ratios (95% confidence limits) for male (vs. female) and age (vs. < 30 years) variables associated with hyponatremia and hypernatremia Na < 136 mmol/l
Na < 126 mmol/l
Risk at presentation (all locations) Male 1.23* (1.19 – 1.27) 1.05 (0.95 – 1.18) 31 – 40 1.08* (1.01 – 1.14) 0.90 (0.67 – 1.22) years 41 – 50 1.04 (0.99 – 1.10) 1.33* (1.01 – 1.69) years 51 – 60 0.88* (0.84 – 0.93) 1.48* (1.19 – 1.84) years 61 – 70 1.06* (1.01 – 1.12) 2.49* (2.05 – 3.03) years 71 – 80 1.39* (1.32 – 1.47) 4.31* (3.57 – 5.21) years >81 1.89* (1.78 – 2.01) 7.66* (6.33 – 9.27) years
Na < 116 mmol/l
Na>145 mmol/l
Na>155 mmol/l
0.83 (0.66 – 1.05) 0.60 (0.24 – 1.46)
1.04 (0.92 – 1.19) 0.96 (0.68 – 1.36)
1.21 (0.66 – 2.24)
1.29 (0.97 – 1.70)
1.06 (0.73 – 100) 1.10 (0.55 – 2.17) 4.25* (1.42 – 12.7) Age 31 – 60 years: 1.40 (0.27 – 7.24) 1.63 (0.47 – 5.64)
1.98* (1.18 – 3.33)
1.26 (0.97 – 1.65)
0.81 (0.19 – 3.38)
2.76* (1.71 – 4.48)
1.93* (1.52 – 2.46)
3.33* (1.19 – 9.34) 1.93 (0.32 – 11.53)
6.65* (4.24 – 10.44)
3.13* (2.48 – 3.96)
10.64* (4.1 – 27.5) 7.12* (1.51 – 33.62)
8.70* (5.44 – 13.90)
7.09* (5.65 – 8.88)
32.3* (12.9 – 81.4) 24.39* (5.53 – 107.45)
1.19* (1.06 – 1.33) 0.92 (0.66 – 1.28)
1.32* (1.04 – 1.69) 0.89 (0.53 – 1.49) 0.98 (0.49 – 1.98) 2.89 (0.97 – 8.61)
1.49* (1.12 – 1.99)
1.93* (1.09 – 3.42) 1.67 (0.47 – 5.94)
3.07* (2.39 – 3.93)
2.53* (1.45 – 4.43) 3.96* (1.37 – 11.43)
4.99* (3.99 – 6.24)
6.54* (4.14 – 10.3) 5.84* (2.19 – 15.58)
7.36* (5.95 – 9.10)
7.00* (4.44 – 11.0) 5.46* (2.02 – 14.81)
Risk of developing during admission (acute hospital) Male 1.14* (1.06 – 1.20) 1.19* (1.02 – 1.39) 0.92 (0.60 – 1.39) 31 – 40 0.96 (0.83 – 1.11) 0.95 (0.60 – 1.51) 0.93 (0.25 – 3.50) years 41 – 50 1.54* (1.35 – 1.74) 1.83* (1.25 – 2.67) 0.94 (0.25 – 3.55) years 51 – 60 2.53* (2.24 – 2.84) 3.67* (2.63 – 5.12) 4.08* (1.64 – 10.17) years 61 – 70 3.80* (3.40 – 4.23) 6.04* (4.46 – 8.17) 4.39* (1.79 – 10.75) years 71 – 80 4.74* (4.27 – 5.26) 7.76* (5.79 – 10.40) 9.70* (4.38 – 21.48) years >81 5.81* (5.22 – 6.48) 10.88* (8.13 – 14.57) 12.78* (5.77 – 28.30) years * p < 0.05.
Na>165 mmol/l
12.96* (10.6 – 15.9) 12.88* (8.3 – 20.0) 7.53* (2.81 – 20.21)
R.C. Hawkins / Clinica Chimica Acta 337 (2003) 169–172
Microsoft Access for sorting. The initial data set was filtered to include only samples with known age, gender, and location. To identify the pattern of dysnatremia at admission/initial presentation, only the initial Na result for any given individual was used. Additionally for the hospital acute care group (inpatient and emergency department), separate filtering to identify the overall lowest and highest Na of multiple samples from a given individual was performed to allow identification of development of dysnatremia during admission. Statistical analysis was performed using SPSS v. 11.0 (SPSS). For multiple logistic regression, gender and increasing age were independent variables, and hyponatremia or hypernatremia were the dependent variables. Calculated odds ratios used females as the gender reference group and patients < 30 years as the age reference group.
3. Results Over the 2-year period, 306,140 samples were analysed for serum Na, of which 303,577 samples (120,137 patients) had age, gender, and location recorded. The average age was 55 years with 43% aged >60 years and 23% aged >70 years. A total of 36% (average age 53.4 years) of patients were from acute hospital care (inpatient and emergency department), 44% (53.8 y) from ambulatory hospital care (outpatient), and 20% (59.6 years) from communitybased care (polyclinics). The prevalence of dysnatremia in different patient groups at initial presentation to a health care provider is shown in Table 1. For the acute hospital care group, overall prevalence of dysnatremia (either on or following admission) was Na < 136 mmol/l: 42.6% (28.2% on admission, 14.4% following admission); < 126: 6.2% (2.5%, 3.7%); < 116: 1.2% (0.5%, 0.7%); >145: 6.9% (1.4%, 5.5%); >155: 1.7% (0.2%, 1.5%); >165: 0.4% (0.06%, 0.34%) respectively. The odds ratios for gender and increasing age associated with dysnatremia at presentation (all populations) and developing dysnatremia following admission (acute hospital population) are shown in Table 2 (age bands with insufficient cases are combined). Odds ratios >1.0 signify increased risk compared to the reference groups (females for the male odds ratios and subjects < 30 years for the age-related
171
odds ratios). The odds ratios can be combined with the prevalence data in Table 1 to calculate relative risk if required [11].
4. Discussion These results provide quantification of the prevalence of dysnatremia in different populations, confirming hyponatremia as a common but generally mild condition, which is often present on admission [12], and hypernatremia as uncommon and usually developing after admission [4]. Inspection of the few comparable prevalence figures published suggest similar rates might be seen in Western populations. Miller et al. found a hyponatremic preva1ence of 18% in nursing home residents >60 years (similar to the 18.2% for patients >60 years in this study), while Long et al. found an adult inpatient hypernatremic (Na>150 mmol/l) prevalence of 0.3% (similar to the 0.22%, >155 mmol/l seen here) [4,6]. The study shows age to be a strong independent risk factor for both presenting with or developing dysnatremia, especially hypernatremia. Aging is associated with a decreased capacity to cope with environmental, disease-related, and iatrogenic (especially drug-related) stresses in sodium and water balance, due to reductions in total body water, thirst sensation, renal responsiveness to ADH, and glomerular filtration rate [13]. This places elderly individuals at increased risk of both hypo- and hypernatremia. In this study, male gender is a weak risk factor for mild hyponatremia only, and gender was otherwise unimportant. Previous work has suggested that the importance of female gender as a risk factor for hyponatremia is confounded by low body weight [14]. The lack of gender difference in this study may reflect a smaller difference in body weight between men and women in this mixed Asian ethnicity population than the predominantly Caucasian populations studied previously. Ethnic differences in body composition, such as increased body fat in Asians compared to Caucasians [15], are documented and may also be a contributing factor. With almost 50% of inpatients experiencing hyponatremia or hypernatremia during their admission, the findings of this study should remind both the laboratory and medical staff of the frequency and risk factors for these conditions.
172
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References [1] Borroni G, Maggi A, Sangiovanni A, Cazzaniga M, Salerno F. Clinical relevance of hyponatraemia for the hospital outcome of cirrhotic patients. Dig Liver Dis 2000;32:605 – 10. [2] Crook MA, Velauthar U, Moran L, Griffiths W. Review of investigation and management of severe hyponatraemia in a hospital population. Ann Clin Biochem 1999;36(Pt 2):158 – 62. [3] Polderman KH, Schreuder WO, Strack van Schijndel RJ, Thijs LG. Hypernatremia in the intensive care unit: an indicator of quality of care? Crit Care Med 1999;27:1105 – 8. [4] Long CA, Marin P, Bayer AJ, Shetty HG, Pathy MS. Hypernatraemia in an adult in-patient population. Postgrad Med J 1991;67:643 – 5. [5] Kirby D, Harrigan S, Ames D. Hyponatraemia in elderly psychiatric patients treated with Selective Serotonin Reuptake Inhibitors and venlafaxine: a retrospective controlled study in an inpatient unit. Int J Geriatr Psychiatry 2002;17:231 – 7. [6] Miller M, Morley JE, Rubenstein LZ. Hyponatremia in a nursing home population. J Am Geriatr Soc 1995;43:1410 – 3. [7] Madhusoodanan S, Bogunovic OJ, Moise D, Brenner R, Markowitz S, Sotelo J. Hyponatraemia associated with psychotropic medications. A review of the literature and spontaneous reports. Adverse Drug React Toxicol Rev 2002;21:17 – 29.
[8] Sonnenblick M, Friedlander Y, Rosin AJ. Diuretic-induced severe hyponatremia. Review and analysis of 129 reported patients. Chest 1993;103:601 – 6. [9] Davis DP, Videen JS, Marino A, et al. Exercise-associated hyponatremia in marathon runners: a two-year experience. J Emerg Med 2001;21:47 – 57. [10] Rosholm JU, Nybo H, Andersen Ranberg K, et al. Hyponatraemia in very old nonhospitalised people: association with drug use. Drugs Aging 2002;19:685 – 93. [11] Zhang J, Yu K. What’s the relative risk? JAMA 1998;280: 1690 – 1. [12] Erasmus RT, Matsha TE. The frequency, aetiology and outcome of severe hyponatraemia in adult hospitalised patients. Cent Afr J Med 1998;44:154 – 8. [13] Kugler JP, Hustead T. Hyponatremia and hypernatremia in the elderly. Am Fam Phys 2000;61:3623 – 30. [14] Wilkinson TJ, Begg EJ, Winter AC, Sainsbury R. Incidence and risk factors for hyponatraemia following treatment with fluoxetine or paroxetine in elderly people. Br J Clin Pharmacol 1999;47:211 – 7. [15] Deurenberg P, Deurenberg-Yap M, Guricci S. Asians are different from Caucasians and from each other in their body mass index/body fat per cent relationship. Obes Rev 2002;3:141 – 6.
Age and gender as risk factors for hyponatremia and hypernatremia Robert C. Hawkins * Department of Pathology and Laboratory Medicine, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore Received 28 July 2003; received in revised form 8 August 2003; accepted 12 August 2003
Abstract Background: This study assesses gender and age as independent risk factors for hypo- and hypernatremia and describes the prevalence of hypo- and hypernatremia in different population groups. Methods: Details of all serum Na results with accompanying patient demographics for 2 years were downloaded from the laboratory database into Microsoft Access for multiple logistic regression analysis using SPSS. Female gender and age < 30 years were the reference groups. Results: Data from 303,577 samples on 120,137 patients were available for analysis. Prevalence at initial presentation to a health care provider of Na < 136, < 116, >145, and >165 mmol/l were for acute hospital care patients: 28.2%, 0.49%, 1.43%, and 0.06%; ambulatory hospital care: 21%, 0.17%, 0.53%, and 0.01%; community care: 7.2%, 0.03%, 0.72%, and < 0.01%. Age odds ratios rose with increasing age to 1.89 and 8.70 (Na < 136 and < 116 mmol/l) and 7.09 and 24.39 (Na>145 and >165 mmol/l, respectively) for age >81 years. Male gender was a mild risk factor for Na < 136 mmol/l and was otherwise unimportant. Conclusions: Hyponatremia is a common but generally mild condition while hypernatremia is uncommon. Increasing age is a strong independent risk factor for both hypo- and hypernatremia. Gender is not an important risk factor for disturbances of serum Na concentration. D 2003 Elsevier B.V. All rights reserved. Keywords: Age; Gender; Hypernatremia; Hyponatremia; Risk factor
1. Introduction There is little agreement on the prevalence of disturbances of sodium balance or on the importance of gender and age as markers of risk. Previous estimates of hyponatremia of 0.2 –29.8% [1,2] and hypernatremia of 0.3 – 8.9% [3,4] have varied with the differences in the definitions of the disturbance and the population surveyed. Most studies on hyponatremia have examined elderly and/or psychiatric populations with particular emphasis on psychotropic * Tel.: +65-6-3578943; fax: +65-6-2536507. E-mail address: [email protected] (R.C. Hawkins). 0009-8981/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cccn.2003.08.001
medications as a cause of hyponatremia [5,6]. Little has been published on the comparative prevalence of hypo- and hypernatremia in different patient populations (hospital or community). Female gender has been suggested as a risk factor for psychotropic [7], diuretic [8], and exercise-induced [9] hyponatremia. While some studies show increasing age to be a risk factor [7], others show no association with age [8,10]. This study assesses gender and age as independent risk factors for hypo- and hypernatremia in a mixed community/hospital population and describes the prevalence of different levels of dysnatremia (hyponatremia and hypernatremia) in different population groups.
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R.C. Hawkins / Clinica Chimica Acta 337 (2003) 169–172
Table 1 Prevalence of hyponatremia and hypernatremia in different patient groups at presentation Acute Ambulatory Community Age>60 Age>70 hospital hospital care (%) years years care (%) care (%) (%) (%) Na < 136 mmol/l Na < 135 mmol/l Na < 126 mmol/l Na < 116 mmol/l Na >145 mmol/l Na >155 mmol/l Na >165 mmol/l
28.2
21
7.2
23.6
27.3
22.1
14.7
4.3
18.2
21.7
2.6
0.96
0.14
2.2
2.9
0.49
0.17
0.03
0.44
0.6
1.43
0.53
0.72
1.3
1.7
0.22
0.05
0.01
0.2
0.3
0.06
0.01
< 0.01
0.05
0.09
2. Materials and methods Tan Tock Seng Hospital Biochemistry Laboratory services a 1200-bed acute care hospital and its attendant outpatient services, and three community-based primary care polyclinics. All patients are admitted to the hospital via the emergency department. Serum Na measurements are performed on two Roche 917 clinical chemistry analysers using indirect ion-selective electrode technology. A 95% reference interval of 136 – 145 mmol/l was established in 1999 based on 560 normoglycemic multiphasic health screen samples. There were no significant differences in Na concentration or age between male and female reference individuals (mean age 47 years). Details of all serum Na requests for the last 2 years (2001 –2002) were downloaded from the laboratory information system into Microsoft Excel and then
Table 2 Odds ratios (95% confidence limits) for male (vs. female) and age (vs. < 30 years) variables associated with hyponatremia and hypernatremia Na < 136 mmol/l
Na < 126 mmol/l
Risk at presentation (all locations) Male 1.23* (1.19 – 1.27) 1.05 (0.95 – 1.18) 31 – 40 1.08* (1.01 – 1.14) 0.90 (0.67 – 1.22) years 41 – 50 1.04 (0.99 – 1.10) 1.33* (1.01 – 1.69) years 51 – 60 0.88* (0.84 – 0.93) 1.48* (1.19 – 1.84) years 61 – 70 1.06* (1.01 – 1.12) 2.49* (2.05 – 3.03) years 71 – 80 1.39* (1.32 – 1.47) 4.31* (3.57 – 5.21) years >81 1.89* (1.78 – 2.01) 7.66* (6.33 – 9.27) years
Na < 116 mmol/l
Na>145 mmol/l
Na>155 mmol/l
0.83 (0.66 – 1.05) 0.60 (0.24 – 1.46)
1.04 (0.92 – 1.19) 0.96 (0.68 – 1.36)
1.21 (0.66 – 2.24)
1.29 (0.97 – 1.70)
1.06 (0.73 – 100) 1.10 (0.55 – 2.17) 4.25* (1.42 – 12.7) Age 31 – 60 years: 1.40 (0.27 – 7.24) 1.63 (0.47 – 5.64)
1.98* (1.18 – 3.33)
1.26 (0.97 – 1.65)
0.81 (0.19 – 3.38)
2.76* (1.71 – 4.48)
1.93* (1.52 – 2.46)
3.33* (1.19 – 9.34) 1.93 (0.32 – 11.53)
6.65* (4.24 – 10.44)
3.13* (2.48 – 3.96)
10.64* (4.1 – 27.5) 7.12* (1.51 – 33.62)
8.70* (5.44 – 13.90)
7.09* (5.65 – 8.88)
32.3* (12.9 – 81.4) 24.39* (5.53 – 107.45)
1.19* (1.06 – 1.33) 0.92 (0.66 – 1.28)
1.32* (1.04 – 1.69) 0.89 (0.53 – 1.49) 0.98 (0.49 – 1.98) 2.89 (0.97 – 8.61)
1.49* (1.12 – 1.99)
1.93* (1.09 – 3.42) 1.67 (0.47 – 5.94)
3.07* (2.39 – 3.93)
2.53* (1.45 – 4.43) 3.96* (1.37 – 11.43)
4.99* (3.99 – 6.24)
6.54* (4.14 – 10.3) 5.84* (2.19 – 15.58)
7.36* (5.95 – 9.10)
7.00* (4.44 – 11.0) 5.46* (2.02 – 14.81)
Risk of developing during admission (acute hospital) Male 1.14* (1.06 – 1.20) 1.19* (1.02 – 1.39) 0.92 (0.60 – 1.39) 31 – 40 0.96 (0.83 – 1.11) 0.95 (0.60 – 1.51) 0.93 (0.25 – 3.50) years 41 – 50 1.54* (1.35 – 1.74) 1.83* (1.25 – 2.67) 0.94 (0.25 – 3.55) years 51 – 60 2.53* (2.24 – 2.84) 3.67* (2.63 – 5.12) 4.08* (1.64 – 10.17) years 61 – 70 3.80* (3.40 – 4.23) 6.04* (4.46 – 8.17) 4.39* (1.79 – 10.75) years 71 – 80 4.74* (4.27 – 5.26) 7.76* (5.79 – 10.40) 9.70* (4.38 – 21.48) years >81 5.81* (5.22 – 6.48) 10.88* (8.13 – 14.57) 12.78* (5.77 – 28.30) years * p < 0.05.
Na>165 mmol/l
12.96* (10.6 – 15.9) 12.88* (8.3 – 20.0) 7.53* (2.81 – 20.21)
R.C. Hawkins / Clinica Chimica Acta 337 (2003) 169–172
Microsoft Access for sorting. The initial data set was filtered to include only samples with known age, gender, and location. To identify the pattern of dysnatremia at admission/initial presentation, only the initial Na result for any given individual was used. Additionally for the hospital acute care group (inpatient and emergency department), separate filtering to identify the overall lowest and highest Na of multiple samples from a given individual was performed to allow identification of development of dysnatremia during admission. Statistical analysis was performed using SPSS v. 11.0 (SPSS). For multiple logistic regression, gender and increasing age were independent variables, and hyponatremia or hypernatremia were the dependent variables. Calculated odds ratios used females as the gender reference group and patients < 30 years as the age reference group.
3. Results Over the 2-year period, 306,140 samples were analysed for serum Na, of which 303,577 samples (120,137 patients) had age, gender, and location recorded. The average age was 55 years with 43% aged >60 years and 23% aged >70 years. A total of 36% (average age 53.4 years) of patients were from acute hospital care (inpatient and emergency department), 44% (53.8 y) from ambulatory hospital care (outpatient), and 20% (59.6 years) from communitybased care (polyclinics). The prevalence of dysnatremia in different patient groups at initial presentation to a health care provider is shown in Table 1. For the acute hospital care group, overall prevalence of dysnatremia (either on or following admission) was Na < 136 mmol/l: 42.6% (28.2% on admission, 14.4% following admission); < 126: 6.2% (2.5%, 3.7%); < 116: 1.2% (0.5%, 0.7%); >145: 6.9% (1.4%, 5.5%); >155: 1.7% (0.2%, 1.5%); >165: 0.4% (0.06%, 0.34%) respectively. The odds ratios for gender and increasing age associated with dysnatremia at presentation (all populations) and developing dysnatremia following admission (acute hospital population) are shown in Table 2 (age bands with insufficient cases are combined). Odds ratios >1.0 signify increased risk compared to the reference groups (females for the male odds ratios and subjects < 30 years for the age-related
171
odds ratios). The odds ratios can be combined with the prevalence data in Table 1 to calculate relative risk if required [11].
4. Discussion These results provide quantification of the prevalence of dysnatremia in different populations, confirming hyponatremia as a common but generally mild condition, which is often present on admission [12], and hypernatremia as uncommon and usually developing after admission [4]. Inspection of the few comparable prevalence figures published suggest similar rates might be seen in Western populations. Miller et al. found a hyponatremic preva1ence of 18% in nursing home residents >60 years (similar to the 18.2% for patients >60 years in this study), while Long et al. found an adult inpatient hypernatremic (Na>150 mmol/l) prevalence of 0.3% (similar to the 0.22%, >155 mmol/l seen here) [4,6]. The study shows age to be a strong independent risk factor for both presenting with or developing dysnatremia, especially hypernatremia. Aging is associated with a decreased capacity to cope with environmental, disease-related, and iatrogenic (especially drug-related) stresses in sodium and water balance, due to reductions in total body water, thirst sensation, renal responsiveness to ADH, and glomerular filtration rate [13]. This places elderly individuals at increased risk of both hypo- and hypernatremia. In this study, male gender is a weak risk factor for mild hyponatremia only, and gender was otherwise unimportant. Previous work has suggested that the importance of female gender as a risk factor for hyponatremia is confounded by low body weight [14]. The lack of gender difference in this study may reflect a smaller difference in body weight between men and women in this mixed Asian ethnicity population than the predominantly Caucasian populations studied previously. Ethnic differences in body composition, such as increased body fat in Asians compared to Caucasians [15], are documented and may also be a contributing factor. With almost 50% of inpatients experiencing hyponatremia or hypernatremia during their admission, the findings of this study should remind both the laboratory and medical staff of the frequency and risk factors for these conditions.
172
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References [1] Borroni G, Maggi A, Sangiovanni A, Cazzaniga M, Salerno F. Clinical relevance of hyponatraemia for the hospital outcome of cirrhotic patients. Dig Liver Dis 2000;32:605 – 10. [2] Crook MA, Velauthar U, Moran L, Griffiths W. Review of investigation and management of severe hyponatraemia in a hospital population. Ann Clin Biochem 1999;36(Pt 2):158 – 62. [3] Polderman KH, Schreuder WO, Strack van Schijndel RJ, Thijs LG. Hypernatremia in the intensive care unit: an indicator of quality of care? Crit Care Med 1999;27:1105 – 8. [4] Long CA, Marin P, Bayer AJ, Shetty HG, Pathy MS. Hypernatraemia in an adult in-patient population. Postgrad Med J 1991;67:643 – 5. [5] Kirby D, Harrigan S, Ames D. Hyponatraemia in elderly psychiatric patients treated with Selective Serotonin Reuptake Inhibitors and venlafaxine: a retrospective controlled study in an inpatient unit. Int J Geriatr Psychiatry 2002;17:231 – 7. [6] Miller M, Morley JE, Rubenstein LZ. Hyponatremia in a nursing home population. J Am Geriatr Soc 1995;43:1410 – 3. [7] Madhusoodanan S, Bogunovic OJ, Moise D, Brenner R, Markowitz S, Sotelo J. Hyponatraemia associated with psychotropic medications. A review of the literature and spontaneous reports. Adverse Drug React Toxicol Rev 2002;21:17 – 29.
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