Phase 2 Variables and Parameters

Although the study’s objective was eminently descriptive, a logistic regression model was fitted to estimate individual risk for vascular damage as a function of the variables described previously— except for frank cardiovascular or cerebrovascular disease. Using an ROC (receiver operating characteristic) curve, a separability index was estimated for subjects with microalbuminuria confirmed with ACR by nephelometry. After fitting the logistic regression model, a Z-score was calculated for BMI, given the well-known lack of linearity of BMI and risk of CKD. Squared Z-scores were used to account for higher risks at the tails of the distribution. Finally, a regression tree was used to evaluate the gradient for microalbuminuria positivity according to ranges of variation in estimated risk.

RESULTS
Older adults (aged ≥60 years) predominated in the cohort studied in Phase 2. Half of this cohort was hypertensive, and a high percentage regularly used NSAIDs. Two-thirds were overweight or obese, one-third had hypertriglyceridemia, and one-fifth were diabetic (Table 1).

Using the same Micral-Test in a single urine sample as in Phase 1, persistent microalbuminuria was confirmed in only 18% of cases diagnosed five years earlier as presumptive positives. The distribution of albuminuria was as follows: 10.2% in the 20–49 mg/L range; 4.0% in the 50– 99 mg/L range; and 3.6% in the ≥100 mg/L range. Estimated microalbuminuria confirmation in the initial at-risk adult population was 4.4%, or 1.2% of the total adult population screened. Positivity was even lower (2.9% of at-risk adults) when using ARC by nephelometry (Table 2).

Conceptually, all individuals with persistent microalbuminuria are considered to have CKD. The current definition and classification of CKD, proposed in 2002 by the National Kidney Foundation’s Kidney Disease Outcome Quality Initiative (KDOQI), says that persistence of microalbuminuria for a period equal to or greater than three months is synonymous with chronic kidney disease.[17]

Analysis of microalbuminuria association with risk markers and factors gave the following results: risk increases monotonically with age; men are at slightly higher risk than women (Table 3). Underweight is the primary risk for positive microalbuminuria, the relationship between weight and risk resembling an inverted J-shape. With quadratic transformation, BMI Z scores show a positive relationship to risk after adjusting for other risk factors, supporting the preceding presumption. Positive microalbuminuria rates were higher in individuals with elevated cholesterol and triglycerides than in those with normal values.

Both smokers and ex-smokers had higher microalbuminuria levels than non-smokers. A similar association was seen in regular NSAID users.

Individuals with a history of hypertension showed a positive microalbuminuria rate of 14.4% compared to 9.7% of those with no such history. Blood pressure readings taken in Phase 2 indicate that as blood pressure rose, so did positive microalbuminuria; even prehypertension is distinguished from normotension and this is even more pronounced for Stages 1 and 2 hypertension (Table 4). Of subjects diagnosed with diabetes, 20.9% tested positive for microalbuminuria, compared to 9.9% for non-diabetics. Elevated fasting glucose levels were associated with higher microalbuminuria rates, compared to normal glycemia levels, and the rate was even higher for glycemia levels in the diabetic range (Table 4).

A history of ischemic heart disease was associated with higher microalbuminuria positivity rates; this was even truer for history of cerebrovascular disease, at levels greater than diabetes (Table 4).

The presence of any of the chronic atherosclerotic vascular diseases was associated with a greater prevalence of microalbuminuria (Table 4).

The fitted logistic regression model shows the relevance of age, elevated triglycerides and smoking as factors that predispose to microalbuminuria. The large sample sizes may be responsible for the appearance of marginal effects. What is important, nevertheless, is that the model enables estimating an individual risk score that exhibits acceptable separability between positive and non-positive cases (Tables 5, 6).

Furthermore, a regression tree was fitted that shows the properties of this estimated score for determining a clear risk gradient, with positivity distributed as follows (Figure 1):

• <0.08: 5.2%
• 0.08–0.12: 10.7%
• >0.12: 18.7%

Among missing cases (those with a missing value in any of the predictors) the positivity rate was 29.4%.

DISCUSSION
The study cohort was comprised of patients with high prevalence of vascular and renal risk factors. In the general population, the natural evolution of CKD and other chronic atherosclerotic vascular diseases toward their terminal stages has its beginnings in modifiable risk factors that initiate disease and cause it to progress. These factors act synergistically with an individual’s personal susceptibility (non-modifiable risk factors).[20] Confirmation of persistent microalbuminuria in this study enabled stratification of population and individuals at risk, an important public health element that permits more efficient and effective use of resources for population health care.

In Phase 2, percentages of microalbuminuria estimated in the total adult population (1.2%) were lower than those obtained in Phase 1 (6.8%). These data reaffirm the need to have at least two positive samples, taken at an interval of at least two weeks, for diagnosis of persistent microalbuminuria.[17] Unfortunately, it was logistically impossible to carry out confirmatory testing within Phase 1. In addition, the difference obtained over several years between samplings may have been influenced by the fact that in Phase 2, all testing was centralized and was performed by specialized personnel, resulting in less inter-rater variability than in Phase I. Studies done on general population samples in other countries, confirmed in shorter time periods, found higher levels than those obtained by this study: AusDiab (Australia): 6.0%;[22] PREVEND (Holland): 7.2%;[23] NHANES III (USA):10.5%.[24]

Table 5: Logistic Regression Model for Individual Risk of Microalbuminuria (ACR By Nephelometry) by Risk Factor

a Squared Z-score of body mass index b Ordinal categorical variable for cholesterol c Ordinal categorical variable for triglycerides

Various factors can affect albuminuria measurements. Microalbuminuria is a marker of microvascular damage and systemic inflammation; the latter can be due to acute or chronic inflammatory conditions, including systemic infections, arthritis, colitis, hepatitis, gastritis, dermatitis, fever, heart failure, ketosis, hyperglycemia, hemodynamic stress (such as from exercise) and urinary tract infection. All these conditions can increase albuminuria levels, or skew results from the presence of hematuria or leukocyturia.[10,25] In the study cohort, an attempt was made to reduce to a minimum the influence of these factors. Definitive diagnosis of microalbuminuria should be evaluated over time and based on comprehensive clinical assessment of each patient. However, once microalbuminuria is confirmed, it is of great value in stratification of risk and in determining patient prognosis.

The presence and level of albuminuria have important diagnostic and prognostic significance for CKD and its cardiovascular complications.[ 26] The KDIGO Controversies Conference (London, 2009) recommended adding ACR (mg/g) (A1: <30; A2: 30–300; and A3: >300) to the CKD staging system.[27,28]

Results of this study confirm progression of atherosclerotic vascular damage parallel to aging and highlight the importance of microalbuminuria for detecting older adults at greater vascular risk. One of the characteristics of the global pandemic of chronic vascular diseases is their greater prevalence at most advanced ages.[29,30] In the United Kingdom, incidence in persons aged >65 years is >350 per 1 million population,[31] while in the United States, in 2007, it was 1687 per 1 million population for the group aged ≥75 years. The mean age of patients who began dialysis in the US that year was 64.7 years.[3]

The slightly higher microalbuminuria prevalence in men could be interpreted as evidence of worse evolution, but a more comprehensive assessment of other markers and GFR is needed. Incidence, prevalence and evolution by sex are controversial. In Norway, Eriksen studied disease progression in 58,000 Stage 3 CKD patients for 10 years and found a slower decline in GFR, better evolution and higher renal survival in women than in men.[32] Incidence of dialysis is greater in men than women in the United States,[3] Japan,[33] and also in Cuba.[34] However, in a prospective study of risk factors for CKD in Maryland, USA, the hazard ratio for developing the disease was 2.5 among women compared to 1.4 among men.[35] Glassock refers to research that found a female-to-male ratio of 1.75:1 in Stage 3 CKD.[36] The difference among these studies could be due to differing prevalence of risk factors in the two sexes, primarily of diabetes and hypertension.

Underweight individuals had higher microalbuminuria positivity. One explanation could be that they were older adults who had lost lean body mass, stunted type-1 diabetics, or patients with advanced CKD with accompanying malnutrition. No difference in microalbuminuria prevalence was found between normal weight individuals and those who were overweight or obese, which is not consistent with previous reports.[25]

Vascular and renal damage from dyslipidemia was validated in the study cases by the greater prevalence of microalbuminuria in patients with elevated cholesterol and triglyceride levels. Dyslipidemia is considered one of the traditional risk factors for development of atherosclerosis, cardio-cerebral vascular disease and CKD.[20] Dyslipidemia activates proinflammatory genes that lead to endothelial dysfunction, oxidative stress, inflammation and defective vasodilation of the micro- and macrovasculature.[37]

Smoking produces substances that activate proinflammatory, prothrombotic and vasoconstrictive mediators that accelerate vascular and renal damage.[38] The fact that ex-smokers have a higher rate of positivity than active smokers may be because the ex-smoker group should include older adults and therefore, individuals with greater associated risks. Additionally, clinical practice shows that smoking cessation is more likely among individuals who have had some adverse health event that has left more extensive vascular injury; in addition, once these vascular lesions are established, their regression is unlikely, even with smoking cessation.

A high proportion of the study population regularly uses NSAIDs and this study shows their vascular-renal toxicity. When renal function is normal, the role of prostaglandin synthesis in maintaining renal hemodynamics is relatively limited. However, when renal perfusion is reduced, as occurs in several forms of cardio-renal disease, dehydration and elderly kidney, adequate renal prostaglandin synthesis becomes the main factor in compensatory activation of renal hemodynamics. In these circumstances, NSAID-induced inhibition of renal prostaglandin synthesis can lead to various renal dysfunction syndromes.[39]

Microalbuminuria exhibits a linear relationship with hypertension as an ordinal indicator. Hypertension is an independent risk factor for cardiovascular disease and for CKD. The frequency of microalbuminuria in study participants was lower than that described in a Spanish study of essential hypertensives (31.3%).[40] The authors point out that microalbuminuria is the renal manifestation of a generalized disorder, characterized by an increase in endothelial permeability, fibrinolysis and coagulation, and the activation of inflammatory processes. Testing for microalbuminuria is the first line for identifying hypertensive patients at high cardiovascular risk, providing the basis for rational use of more expensive therapeutic regimens.

The study cases showed a marked increase in microalbuminuria in the diabetic population and even in individuals with elevated fasting glucose, demonstrating a linear progression of risk. A 15-year follow-up study in the United Kingdom of 4000 diabetics found that 40% of participants developed microalbuminuria and 30% developed renal insufficiency. However, 50% of the latter group did not have preceding microalbuminuria.[41] This suggests that the absence of microalbuminuria does not rule out development of kidney failure among diabetics, because, among other things, diabetics are exposed to other, non-diabetic CKD. This is the reason why the term diabetic nephropathy has been replaced by diabetic renal disease and the term diabetic glomerulopathy has been reserved for demonstration by renal biopsy.[42]

The elevated prevalence of microalbuminuria in patients with a history of ischemic heart disease reflects that albuminuria is a continuous risk for occurrence of cardiovascular events, even below the cutoff points for microalbuminuria.[43] Albuminuria/proteinuria predict cardiovascular and non-cardiovascular mortality in the general population.

Patients with a history of cerebrovascular disease exhibited the highest prevalence of microalbuminuria among all cases studied, which could be an indication that underlying vascular damage is greater. There is a connection between CKD and cognitive phenomena and stroke. The development of cerebrovascular disease is a process involving transition from risk factors for cardiovascular disease, which include CKD, to clinical manifestations of cognitive impairment and stroke. It is suggested that albuminuria and GFR reduction can be used to identify patients who might benefit from more detailed investigations of cognitive deterioration.[44] Risk of dementia increases in the presence of albuminuria. These findings suggest a shared susceptibility for microvascular disease in the brain and kidney in older adults.[45]

CONCLUSIONS
Microalbuminuria is a marker for risk and for early renal and cardiocerebral vascular damage. In the at-risk study cohort, confirmation of microalbuminuria after five years showed low rates of persistence. In cases with persistent microalbuminuria, there was a correlation with all risk factors, except overweight and obesity. Persistent microalbuminuria reveals multiple risk markers and factors. Use of microalbuminuria in screening for chronic kidney disease reveals presence of other chronic atherosclerotic vascular diseases. Persistence of microalbuminuria in patients with vascular risk factors places them at higher risk, indicating early organ damage, which may be reversible. Stratification of risk enables better medical care for patients, more rational use of public health resources, more effective preventive and curative actions, and better orientation of intersectoral measures focused on the individual, the family and the community.

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THE AUTHORS
Raúl Herrera Valdés (Corresponding author: raul.herrera@infomed.sld.cu), nephrologist. Full professor and distinguished researcher, Medical University of Havana and Nephrology Institute, Havana, Cuba.

Miguel Almaguer López, nephrologist. Associate Professor and Senior Researcher, Medical University of Havana, Havana, Cuba. Chief of the Preventive Nephrology Department, Nephrology Institute, Havana, Cuba.

OJosé Chipi Cabrera, nephrologist. Associate Professor, Héroes del Baire Hospital, Nueva Gerona, Isle of Youth Special Municipality, Cuba.

Orquídia Martínez Soto, nephrologist. Chief of the Nephrology Service, Héroes del Baire Hospital, Nueva Gerona, Isle of Youth Special Municipality, Cuba

Jorge Bacallao Gallestey, mathematician and biostatistician. Full professor, Medical University of Havana, Atherosclerosis Research and Reference Center, Havana, Cuba

Néstor Rodríguez Triana, information technologist. Information Technology Department, Héroes del Baire Hospital, Nueva Gerona, Isle of Youth Special Municipality, Cuba.

Miriam de la Caridad Abreu Correa, clinical laboratory scientist. Adjunct professor, Héroes del Baire Hospital, Nueva Gerona, Isle of Youth Special Municipality, Cuba.

Odeime Fariña Martínez, clinical laboratory scientist. Instructor, Héroes del Baire Hospital, Nueva Gerona, Isle of Youth Special Municipality, Cuba.

María del Carmen Roche Gutiérrez, specialist in family medicine and clinical laboratory science. Instructor, Héroes del Baire Hospital, Nueva Gerona, Isle of Youth Special Municipality, Cuba.