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increased mortality does not prove causality
- Effrossyni Gkrania-Klotsas (14 July 2006)
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MRSA bacteraemia and mortality: the emerging role of coexisting risk factors
- George I. Varughese, Abd A. Tahrani, John H.B. Scarpello (6 August 2006)
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MRSA bacteraemia in Oxfordshire - lessons for African health ministries
- Sam Wystan Mhlongo, Acquira Mbokazi, Patrick Maduna (14 August 2006)
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Effrossyni Gkrania-Klotsas, Infectious Diseases Physician Addenbrooke's Hospital, Cambridge, CB2 2QQ
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1. I believe that this study was not a cohort study as such, since it was not a prospective observational study but rather a retrospective record review 2. The study analyzed the episodes of staphylococcus bacteraemia rather than the first episode of staphylococcus bacteraemia in each patient only: without a doubt, a few patients must have had repeatedly positive blood cultures with the same pathogen and probably more so in the MRSA group. It is not clear from the study what proportion of the 441 bacteraemia episodes included in the study might have been from the same patients, something that might have biased the outcomes in the MRSA or the MSSA group or both. 3. The study covered a period of seven years: within these seven years medical advances have certainly occurred, that might have affected both the occurrence of bacteraemia (cohorting, intravenous line care etc) and also the outcome of it (more antibiotics available, better culture systems etc). Stratifying the data to six-month blocks eliminates some but not all of this bias. 4. The importance of comorbidities actually being responsible for this effect can not be underestimated: the studies quoted have not adequately excluded comorbid illness as a possible associated factor. • The study by McClelland et al . did not use a comorbidity index but
artificially separated patients to over 65 and under 60. Different models
that are not presented in the study were constructed, some of them using
the number of comorbid conditions instead of assigning a weight to each
one of them.
5. Even if the difference in mortality is true, we have to be reminded that it does not prove causality: once colonization of the anterior nose with Staphylococcus aureus has happened, autoinfection happens more commonly in MRSA-colonized patients than in MSSA colonized patients because intercurrent antibiotic treatment eliminates one but not the other (Archer G) . Of note that this notion has been recently challenged but the demonstration of intracellular reservoirs Staphylococcus aureus in the nasal mucosa (Clement S). McClelland RS et al. Staphylococcus aureus bacteremia among elderly vs younger adult patients:comparison of clinical features and mortality. Arch Intern Med. 1999 Jun 14;159(11):1244-7. Blot SI, Vandewoude KH, Hoste EA, Colardyn FA. Outcome and attributable mortality in critically Ill patients with bacteremia involving methicillin-susceptible and methicillin-resistant Staphylococcus aureus. Arch Intern Med. 2002 Oct 28;162(19):2229-35. Archer G, Climo M. Staphylococcus aureus bacteremia—consider the source. N Engl J Med. 2001;344:55–56. Clement S et al. Evidence of an intracellular reservoir in the nasal mucosa of patients with recurrent Staphylococcus aureus rhinosinusitis. J Infect Dis. 2005 Sep 15;192(6):1023-8. Epub 2005 Aug 9. Competing interests: None declared |
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George I. Varughese, Specialist Registrar DEPARTMENT OF DIABETES, ENDOCRINOLOGY & GENERAL (INTERNAL) MEDICINE, Abd A. Tahrani, John H.B. Scarpello
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We read with interest the study by Wyllie et al analysing the possible impact of MRSA bacteraemia on short-term mortality [1]. Although they have alluded to the fact that their study is limited by the lack of data on coexisting risk factors for death, this is pertinent in the context of MRSA bacteraemia and indeed very crucial when interpreting the results of their evaluation. As an illustrative example, we had assessed the relationship of mortality in hospital patients with MRSA bacteraemia to the presence of hyperglycaemia on admission during a similar time period (2000-2002) amongst the 309 patients aged 16 years or above with MRSA bacteraemia in our centre and found that mortality was significantly more in the highest tertile of blood glucose distributions [2,3]. This has particular inference, if we take into consideration other commonly associated risk factors for vascular events such as hypertension [4]. Of note, increased screening and surveillance for MRSA was gaining momentum during this period. Wyllie et al have demonstrated that a higher proportion (n=97) of the patients at risk of death within 30 days of diagnosing MRSA bacteraemia were amongst those discharged from Medicine [1]. In view of the global prevalence of chronic illnesses such as diabetes mellitus and the projections for 2030 [5], the results of such retrospective analyses should be interpreted cautiously. 1. Wyllie DH, Crook DW, Peto TEA. Mortality after Staphylococcus aureus bacteraemia in two hospitals in Oxfordshire, 1997-2003: cohort study BMJ 2006; 333: 281-284. 2. Varughese GI, Miltsios K, Buch HN, Orendi JM, Scarpello JH. Mortality rates in hospital patients with hyperglycaemia and MRSA bacteraemia. Br J Diabetes Vasc Dis 2006; 6: 42-44. 3. Varughese GI, Miltsios K, Barton DM, Buch HN, Orendi JM, Scarpello JH. An audit of laboratory blood glucose determinations and their relation to mortality in hospital in-patients with MRSA bacteraemia. Diabetic Medicine 2004; 21(Supp. 2): P192. 4. Varughese GI, Patel JV, Lip GY. Blood pressure control in the setting of diabetes mellitus: new targets, new hope for improvement? J Hum Hypertens 2006 Apr 13; [Epub ahead of print]. 5. Wild S, Roglic G, Green A, Sicree R, King H. Global prevalence of diabetes: estimates for the year 2000 and projections for 2030. Diab Care 2004; 27(5): 1047–1053. George I. Varughese (Specialist Registrar) Abd A. Tahrani (Specialist Registrar) John H.B. Scarpello (Consultant Physician) DEPARTMENT OF DIABETES, ENDOCRINOLOGY & GENERAL (INTERNAL) MEDICINE, UNIVERSITY HOSPITAL OF NORTH STAFFORDSHIRE, STOKE-ON-TRENT ST4 6QG (UK) Competing interests: None declared |
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Sam Wystan Mhlongo, Professor of Family Medicine & Primary Health Care University of Limpopo, Medunsa Campus 0204, South Africa, Acquira Mbokazi, Patrick Maduna
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As family physicians in an academic setting in a 'Third World' country (South Africa), we find the article 'Mortality after Staphylococcus aureus bacteraemia in two acute hospitals in Oxfordshire' by David H Wyllie and colleagues very interesting. We need to point out from the very outset that none of us are microbiologists but that we more than frequently care for patients with intractable bacterial infections at one tertiary hospital and one district hospital. We are aware that in Europe, The Netherlands appears to have found an answer to hospital acquired infections - Methicillin Resistant Staphylococcus Aureus, as indeed published literature shows.(1) We note with interest that in the same issue of The BMJ, consultant microbiologist John Paul while paying tribute to the Dutch approach, he cautions 'To do this properly would require a huge investment in facilities, however, and might take a decade or so to bear fruit.'(2) We agree with this assertion and wish to point out that in impoverished Third World countries, such an approach would for ever remain a pipe dream. Countries of the Third World face numerous acute and chronic problems with regard to caring for the sick. One of the major obstacles is that these countries inherited a model of health care which is largely unsuitable for the care of their populations. That model of health care is what medical social scientists frequently refer to as biomedicine or the engineering model of health care. This model, which continues to be sustained by most health ministries cannot be an answer to poverty, malnutrion, destitution, squalor and extremely poor or absent sanitation. Wyllie and colleagues state 'A key issue is whether bacteraemia was the underlying cause of death in these patients. It is difficult to attribute the cause of death in cases of S aureus bacteraemia because of the many coexisting risk factors for death, such as acute and chronic illness, and a high degree of medical inervention.'(3). In our situation, a large number of patients with bacteraemia are immunocompromised and may have TB as well as numerous opportunistic infections. As family physicians in a Third World country, we strongly advocate the management of patients in their own homes. This, without any doubt would reduce the 'pandemic' of hospital acquired infections. We suggest in the spirit of Alma Ata (1978), that governments divert the enormous resources currently allocated to hospitals to family medicine and primary health care in the communities. References: 1. Bootsma MC, et al, Controlling methicillin-resistant Staphylococcus aureus: quantifying the effects of interventions and rapid diagnostic testing. Proc Natl Acad Sci USA 2006; 103:5620-5 2.John Paul: Surveillance and management of all types of Staphylococcus aureus bacteraemia: BMJ Vol 333, 5 August 2006, 269-270 3.Wylie, David H et al: Mortality after Staphylococcus aureus bacteraemia in two acute hospitals in Oxfordshire, 1997-2003: cohort study: BMJ Vol 333 5 August 2006 Competing interests: None declared |
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