Biomedical subjects

Janez Stare

Publications and source records attributed to Janez Stare.

9 recordsLinked to original sources

PubMed2006

Relative survival analysis in R.

Relative survival techniques are used to compare the survival experience in a study cohort with the one expected should they follow the background population mortality rates. The techniques are especially useful when the cause-specific death information is not accurate or not available since they provide a measure of excess mortality in a group of patients with a certain disease. There are several approaches to modeling relative survival, but there is no widely used statistical package that would incorporate the relevant techniques. The existing software was mostly written by the authors of different methods, in different computer languages and with different requirements for the data input, which makes it almost impossible for a user to choose between available models. We describe our R package relsurv that provides functions for easy and flexible fitting of several relative survival regression models.

Cohort Studies↗

PubMed2006

The risk of lower mental outcome in infantile spasms increases after three weeks of hypsarrhythmia duration.

To assess the correlation between hypsarrhythmia duration and mental outcome in infantile spasms (IS) the medical records of 48 infants with IS were reviewed retrospectively and psychological assessments undertaken at follow-up at the age of 3 to 13 years. We found 18 (38%) cryptogenic IS cases with typical hypsarrhythmia and 30 symptomatic with modified hypsarrhythmia-further classified into 15 cases as multifocal, 10 as pseudoperiodic and 5 as unilateral hypsarrhythmia. A short treatment lag (one to two weeks) occurred in 25, three to four weeks in 10 cases. Spasms ceased within one month after treatment in 23 infants. At follow-up 15 children had normal mental outcome (borderline included). A correlation between hypsarrhythmia duration longer than three weeks and lower mental outcome was found using the logistic regression model. The duration of hypsarrhythmia represents a sensitive prognostic parameter in IS; the risk of mental retardation increases after three weeks of hypsarrhythmia.

Age of Onset↗

PubMed2006

Peripheral arterial disease and ankle-brachial pressure index as predictors of mortality in residents of Metlika County, Slovenia.

AIM: To test how the presence of peripheral arterial disease predicted mortality of middle-aged and elderly residents of Metlika county, a rural area in southeastern Slovenia. METHODS: In 1987, we interviewed and examined a representative cohort of 646 subjects aged 45-80 years at inclusion without overt coronary or cerebrovascular disease, for cardiovascular risk factors and measured the ankle-brachial pressure index (ABPI). Peripheral arterial disease was defined as ABPI<0.90. The subjects were followed up 15 years or until death. All-cause mortality and cardiovascular mortality were assessed and compared between subjects with and without peripheral arterial disease in a multivariate model. RESULTS: There were 580 subjects with normal ABPI and 66 subjects with peripheral arterial disease, among which 49 were asymptomatic and 17 had intermittent claudication. Because subjects with peripheral arterial disease were on average 10 years older than those without peripheral arterial disease, the mere presence of peripheral arterial disease was not an independent predictor of mortality. However, there was a significant interaction of peripheral arterial disease with age, with a more pronounced adverse prognostic effect of peripheral arterial disease in younger than in older age groups. For a 55-year-old subject with peripheral arterial disease, the hazard ratio of dying from any cause in the follow-up period was 2.44 (95% confidence interval [CI], 1.15-4.96) in comparison to an age-matched subject without peripheral arterial disease, but at 75 years of age, the hazard ratio decreased to only 0.71 (95% CI, 0.46-1.09). For cardiovascular mortality, the hazard ratio in the presence of peripheral arterial disease was 6.05 (95% CI, 1.87-16.27) at 55 years and 0.92 (95% CI, 0.54-1.52) at 75 years. Among patients with peripheral arterial disease, each decrement of ABPI at inclusion by 0.10 significantly increased the cardiovascular mortality after 15 years by 30% (P = 0.038). CONCLUSION: Peripheral arterial disease, even asymptomatic, is an important predictor of adverse cardiovascular prognosis in relatively young patients. Reduced ABPI is a strong, independent predictor of cardiovascular mortality in all patients with peripheral arterial disease.

Aged↗

PubMed2005

Goodness of fit of relative survival models.

Additive regression models are preferred over multiplicative models in the analysis of relative survival data. Such preferences are mainly grounded in practical experience with mostly cancer registries data, where the basic assumption of the additivity of hazards is more likely to be met. Also, the interpretation of coefficients is more meaningful in additive than in multiplicative models. Nonetheless, the question of goodness of fit of the assumed model must still be addressed, and while there is an abundance of methods to check the goodness of fit of multiplicative models, the respective arsenal for additive models is almost empty. We propose here a variety of procedures for testing the null hypothesis of a good fit. These are based on partial residuals defined similarly to Schoenfeld residuals familiar for Cox model diagnostics. The tests have appropriate sizes under the null hypothesis, and good power under different alternatives. We investigate their performance through simulations and apply the methods to data from a study into survival of colon cancer patients.

Adolescent↗

PubMed2005

Explained randomness in proportional hazards models.

A coefficient of explained randomness, analogous to explained variation but for non-linear models, was presented by Kent. The construct hinges upon the notion of Kullback-Leibler information gain. Kent and O'Quigley developed these ideas, obtaining simple, multiple and partial coefficients for the situation of proportional hazards regression. Their approach was based upon the idea of transforming a general proportional hazards model to a specific one of Weibull form. Xu and O'Quigley developed a more direct approach, more in harmony with the semi-parametric nature of the proportional hazards model thereby simplifying inference and allowing, for instance, the use of time dependent covariates. A potential drawback to the coefficient of Xu and O'Quigley is its interpretation as explained randomness in the covariate given time. An investigator might feel that the interpretation of the Kent and O'Quigley coefficient, as a proportion of explained randomness of time given the covariate, is preferable. One purpose of this note is to indicate that, under an independent censoring assumption, the two population coefficients coincide. Thus the simpler inferential setting for Xu and O'Quigley can also be applied to the coefficient of Kent and O'Quigley. Our second purpose is to point out that a sample-based coefficient in common use in the SAS statistical package can be interpreted as an estimate of explained randomness when there is no censoring. When there is censoring the SAS coefficient would not seem satisfactory in that its population counterpart depends on an independent censoring mechanism. However there is a quick fix and we argue in favour of its use.

Antimetabolites, Antineoplastic↗

PubMed2004

A hospital outbreak of Legionella from a contaminated water supply.

The authors performed a cross-sectional epidemiological survey to investigate the source of a hospital Legionella outbreak originating in contaminated water. Water temperature and air humidity were measured around possible contamination sources. A dead-end pipe was found to contain Legionella pneumophila serogroup 1. All individuals who acquired legionellosis had spent at least 30 min within 2 m of the contamination source. Among staff, 41 of 71 were exposed, and 31 of these fell ill. All 7 patients exposed to the contaminated water acquired legionellosis. None of the 94 bed-ridden patients from the same units developed the disease. An aerosol with 60% relative air humidity was formed near the suspect water faucets, but the humidity fell rapidly farther from the water source, suggesting that desiccation decreased the risk of infection. The healthy personnel and patients closest to the source acquired legionellosis, suggesting that risk was related less to compromised patients than to exposure.

Adult↗

PubMed2002

Proportional hazards models with frailties and random effects.

We discuss some of the fundamental concepts underlying the development of frailty and random effects models in survival. One of these fundamental concepts was the idea of a frailty model where each subject has his or her own disposition to failure, their so-called frailty, additional to any effects we wish to quantify via regression. Although the concept of individual frailty can be of value when thinking about how data arise or when interpreting parameter estimates in the context of a fitted model, we argue that the concept is of limited practical value. Individual random effects (frailties), whenever detected, can be made to disappear by elementary model transformation. In consequence, unless we are to take some model form as unassailable, beyond challenge and carved in stone, and if we are to understand the term 'frailty' as referring to individual random effects, then frailty models have no value. Random effects models on the other hand, in which groups of individuals share some common effect, can be used to advantage. Even in this case however, if we are prepared to sacrifice some efficiency, we can avoid complex modelling by using the considerable power already provided by the stratified proportional hazards model. Stratified models and random effects models can both be seen to be particular cases of partially proportional hazards models, a view that gives further insight. The added structure of a random effects model, viewed as a stratified proportional hazards model with some added distributional constraints, will, for group sizes of five or more, provide no more than modest efficiency gains, even when the additional assumptions are exactly true. On the other hand, for moderate to large numbers of very small groups, of sizes two or three, the study of twins being a well known example, the efficiency gains of the random effects model can be far from negligible. For such applications, the case for using random effects models rather than the stratified model is strong. This is especially so in view of the good robustness properties of random effects models. Nonetheless, the simpler analysis, based upon the stratified model, remains valid, albeit making a less efficient use of resources.

Humans↗

PubMed2002

Cell membrane fluidity and prognosis of lung cancer.

BACKGROUND: Membranes of tumor cells have been found to posses higher fluidity than membranes of non-tumor cells. Plasma membrane fluidity is significantly correlated with malignant potential of these cells. METHODS: Seventy-five patients operated on for lung cancer were studied prospectively. During the operation, lung tumor samples were taken from the resected lung for evaluation by electron paramagnetic resonance. The fluidity variable H13, which is proportional to the plasma membrane fluidity, was determined from the electron paramagnetic resonance spectra. The association between H13 and survival was determined by survival analysis using Kaplan-Meier curves and Cox regression. RESULTS: Pathologic TNM stage and the fluidity variable H13 were the only prognostic variables significantly associated with survival time in multivariate proportional hazards regression model. Thus, H13 was shown to be an independent prognostic variable for survival, which was also confirmed by a separate analysis relating the TNM stage and H13. Dividing the patients into two groups, one with an H13 value higher than the median and another with H13 below the median, resulted in significantly different survival curves (p = 0.01). CONCLUSIONS: Patients with high plasma membrane fluidity, indicated by high H13 of the resected lung tumor tissue, seem to have poorer prognosis than those with less fluid membranes. We suggest that the fluidity variable could be used as an independent additional prognostic factor and a tool to identify patients who may be helped by adjuvant postoperative therapy.

Aged↗

PubMed2002

Decline in sex ratio at birth after 10-day war in Slovenia: brief communication.

BACKGROUND: We investigated whether the psychological stress related to a short war (26 June-7 July, 1991) in Slovenia induced changes in fertility, sex ratio at birth and semen quality characteristics. METHODS: Sex ratios [i.e. males/(males + females)] for 4966 births in the general population of Slovenia and separately for 1565 births in the Slovenian capital, Ljubljana, from January-March 1992 were compared with the sex ratio calculated for the same time period in 1991 and 1993. Semen analyses for 38 normozoospermic men attending an outpatient infertility clinic from May to September 1991 were also evaluated. RESULTS: In the general population in Slovenia there was a significant fall in the sex ratio at birth in 1992, compared with 1991 (0.504 versus 0.518; P = 0.03). In Ljubljana, the decline in sex ratio in 1992 was even more pronounced: 0.483 versus 0.537 in 1991 (P = 0.0001) and 0.483 versus 0.516 in 1993 (P = 0.005). A decrease in the proportion of sperm that were progressively motile from 56% before the war to 52% after it (P = 0.01) and of those that were rapidly motile from 40 to 36% (P = 0.01) was observed. CONCLUSIONS: Acute psychological stress in relation to a short war in Slovenia resulted 6 to 9 months later in a decrease in the observed sex ratio at birth. Negative changes in sperm motility may be involved in the sex ratio modifications.

Female↗