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P Leffers

Publications and source records attributed to P Leffers.

46 records · Page 3Linked to original sources
PubMed1992

The influence of referral patterns on the characteristics of diagnostic tests.

The discrimination of a diagnostic test--characterized by sensitivity, specificity, likelihood ratio and ROC curve--may be influenced by referral patterns of general practitioners. Symptoms and test results in particular will affect the probability of referral, while the degree of development of the pathological process directly influences the probability of positive test results. Using numerical examples, we analyse and discuss a few specific situations: (1) referral depends only on symptoms; (2) referral depends both on symptoms and on test results; (3) referral depends only on test results. In the first situation, test characteristics and predictive values are invariant over the strata of symptomatology, while in the third situation the predictive values are unchanged. If there is a positive relationship between positive test results and referral probability, overall sensitivity will increase while specificity and likelihood ratio will decrease. A general representation is given for the evaluation of the direction of change of the likelihood ratio as a function of referral probabilities. The shape of receiver-operating characteristic curves is less sensitive to bias, but at the level of specific cut-off points considerable changes may occur.

Diagnosis↗
PubMed1992

Lyme borreliosis: a very infrequent cause of arthritis of undetermined aetiology in the southern part of The Netherlands.

Seventy-three patients with undiagnosed arthritis of undetermined aetiology, 94 patients with classified arthritis (rheumatoid arthritis, ankylosing spondylitis, etc.) and 70 controls were studied for clinical and serological manifestations of Lyme borreliosis. The patients were recruited from the three rheumatology units in the most southern part of The Netherlands. A clinical diagnosis of possible Lyme borreliosis was made in seven of 73 patients with arthritis of undetermined aetiology, in four of 94 patients with classified arthritis and in one of the controls. A definite diagnosis of Lyme borreliosis could be made in only one patient who belonged to the arthritis of undetermined aetiology group. This patient had erythema migrans, arthritis of the knee joint and showed positive antibodies to B. burgdorferi. In the southern part of The Netherlands, Lyme arthritis does not seem to be a frequent cause of arthritis of undetermined aetiology.

Adolescent↗
PubMed1991

Pregnancy and gallstone disease: an empiric demonstration of the importance of specification of risk periods.

This paper discusses the importance of specification of risk periods in research of the effect of pregnancy on the occurrence of gallstone disease. The authors present data from a case-control study, started in 1983 in Maastricht, The Netherlands. The study comprised 100 acute gallstone cases and 305 general population controls and 41 elective cases and 122 elective radiodiagnosis controls. In a conventional analysis, the rate ratio for one or more pregnancies ever (vs. never) was 1.2 (90% confidence interval 1.02-1.36). In an analysis that discerned several subsequent risk periods, an effect was found only for the risk period up to 5 years after pregnancy (rate ratio, 2.4; 90% confidence interval 1.12-4.96). Beyond this risk period no effect was noted. The results suggest that pregnancy increases the rate of gallstone formation only transiently. Six reasons for specifying risk periods in nonexperimental etiologic studies are discussed. The authors contend that ignoring of risk periods explains part of the variation in the findings of earlier studies on pregnancy and the risk of gallstone disease.

Adult↗
PubMed1991

Does alcohol protect against the formation of gallstones? A demonstration of protopathic bias.

Previous studies have found an inverse relation between alcohol use and clinical gallstone disease, suggestive of a protective effect of alcohol use. However, such an inverse relation may (at least partly) be explained by a reduction of alcohol use because of symptoms related to clinical gallstone disease (protopathic bias). We empirically evaluated the consequences of different designs for the avoidance of such bias in a series of case-control studies. A first study deliberately used a design that is commonly seen in the literature. Cases of clinical gallstone disease, referred to hospital because of symptoms, were contrasted with general population controls. The results suggested an inverse relation between alcohol use and gallstones. Next, three alternative case-control studies were performed using designs that safeguard against protopathic bias. In none of these studies was an association between alcohol use and gallstones found. This demonstrates the probable existence of protopathic bias in case-control studies on alcohol use and gallstones. It is argued that earlier non-experimental studies on this topic were susceptible to such bias. This has most likely led to an overestimation of the protective effect.

Adult↗
PubMed1991

The qualification period.

For randomized trials that do not require prompt onset of therapy, a pre-admission preparation, called the qualification period, can deal with certain customary difficulties in design, analysis, and ethics. For the patients' baseline state, the qualification period can be used to "wash out" effects of previous treatment, to verify admission criteria, and to identify or stratify suitable prognostic indicators. For treatment, the qualification period can be used to test and adjust therapeutic dosages, to exclude placebo responders, and to confirm early responsiveness to long-term active therapy. For intention-to-treat analyses, the qualification period can allow special stratifications for patients who are therapeutically difficult to regulate or who comply imperfectly. By offering the cited improvements as well as a brief "pilot exposure" for each patient, the qualification period can also enhance the "informed consent" and general ethics of a trial.

Clinical Trials as Topic↗
PubMed1991

A survey of vascular access for haemodialysis in The Netherlands.

After nearly 30 years of access surgery for haemodialysis, a survey of vascular access in a large population seemed relevant in order to analyse the state of the art. On 1 January 1988, 2195 patients in 52 centres in The Netherlands (population 14,714,900) were on long-term haemodialysis and 588 on continuous ambulant peritoneal dialysis (CAPD) for renal insufficiency. A questionnaire was mailed out, to which there was 100% response. Of all the patients on long-term haemodialysis, 70% had a Cimino fistula, 22% a graft fistula, and 6% a more proximal arm fistula. The remaining 2% had a Scribner shunt. Of all the graft fistulae, the expanded polytetrafluoroethylene (ePTFE) graft was the one most used (58%), followed by the homologous vein graft (25%), the autologous vein graft (13%), and those made of other materials (4%). Preference for acute access was recorded; catheterisation of the subclavian vein was used almost exclusively in 17 centres, while in 24 centres it was chosen in more than 50% of the cases. The femoral vein was almost always the choice for acute access in 6 centres and it was selected in more than 50% of the cases in 17 centres. Another alternative, the Scribner shunt, was applied in selected cases in 22 of the 52 centres. Percutaneous transluminal angioplasty (PTA), as a method for treating stenoses in vessels used for vascular access, was applied routinely in 5 centres and occasionally in 19 centres.

Adult↗
PubMed1991

The reproducibility of urinalysis using multiple reagent test strips.

Ninety urine samples were examined twice by 3 "observers" (two persons, using only visual observation, and one person using a spectrophotometric analyser) using multiple reagent teststrips. To determine reproducibility, inter- and intra-observer agreement were calculated and expressed as Cohen's kappa and as weighted kappa. The results show negligible intra-observer differences between the visual and spectrophotometric observation. The lack of agreement between inter- and intra-observer urinalysis results, using multiple reagent test strips was disappointing, considering the simplicity of the test procedure. Further improvement of reproducibility, e.g. by enhancing the discoloration of the test pads, is necessary. Reproducibility is not improved by using a spectrophotometric analyser instead of visual reading of the test strips.

Esterases↗