Search challenge 4: strategies

In the comments on search challenge 4, Martin noted:

"Syncope, Vasovagal"[Mesh] AND "Blood Donors"[Mesh] gives 21 hits, the first one (17655598) is an intervention study, indicating that syncope incidence is quite different among subgroups. This make giving a general answer more difficult.

Some data on incidence comes from (16202053), which suggests a 0.9% incidence.
Other studies go for vasovagal reactions (not limiting to syncope) and, of course, give much higher numbers.

Data on the consequence of syncope comes from (11778059).

I tried a search that was a little more broad, which did draw in a few tangential things (like how to prevent/reduce complications such as fainting during or after blood donation) but still didn't yield an overwhelming volume, about 90 citations:

(vasovagal OR vagal OR fainting OR syncope) AND (blood donors OR blood donation)

As I was looking through the articles and a broad Google retrieval, I noted that there was a significant body of older literature that looked very relevant and potentially useful. IndexCat, the online system which allows users to search volumes from the Index-Catalogue of the Library of the Surgeon-General's Office. The database currently contains Series 1-5 of the Index (1880-1961), which includes 61 volumes in the 5 series, covering reports, journal articles and other materials from 1880 to 1961. Some of the WWII literature, with huge blood drives to support the war effort, had great surveillance data on side effects.

A quick overall summary (with a few sample articles thrown in for good measure): The majority of the original studies on this question were done between the 1940s and 1960s. Much of the data was generated from the mass blood drives conducted for WWII. Later articles refer to the earlier ones for incidence/frequency rates and are more concerned with predicting which donors are likely to exhibit negative reactions. Reported incidence rates range from less-than 1% to 10%; it is important to recognize that the definitions of syncope (fainting) varied somewhat from study to study.

Reference #1: Callahan R, Edelman EB, Smith MS, Smith JJ. Study of the incidence and characteristics of blood donor "reactors." Transfusion 1963; 3: 76-82.

This article reports on a retrospective review of the records of the Milwaukee Blood Center for blood donors from 1958 to 1962 with a more detailed analysis of the reactor records for the year 1960. They report an overall donor reaction rate of 4.5 percent with a higher rate seen in females. Reactions include mild, moderate and severe, where actual syncope is considered a 'severe' reaction. Of the reactions for all years, such severe reactions had a frequency of 5.7%. For the year 1960, 6.2% of the reactions were considered to be severe. Note that these results are retrospective and thus are subject to errors in recording and interpretation.

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Reference #2: Moloney WC, Lonnergan LR, McClintock JK. Syncope in Blood Donors. N Engl J Med 1946; 234: 114-118.

This article reports a pair of prospective studies on the frequency and characteristics of syncope in blood donors. The first series was composed of 6,882 donors bled between 1942 and 1943; 162 total cases of syncope were identified with 17 'severe' cases. A severe case is defined as one in which loss of consciousness or convulsions were seen. The second series was composed of 9, 251 donors bled during 1943; of these 527 syncope cases were identified with 158 'severe' cases. The authors examine multiple factors including donor characteristics and environment. They report that younger donors, particularly women, were more prone to fainting.

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Reference #3: Trouern-Trend JJ, Cable RG, Badon SJ, Newman BH, Popovsky MA. A case-controlled multicenter study of vasovagal reactions in blood donors: influence of sex, age, donation status, weight, blood pressure and pulse. Transfusion 1999; 39: 316-320.

This retrospective case-controlled study examined 1,890 blood donors with reported syncope from three U.S. blood centers, 1994-1995. These were compared to case-controls and random population controls in a logistic regression analysis to determine which of several factors correlated with the incidence of syncope. The authors report that age, weight and donation status (first-time or repeat donor) were significant but that sex, blood pressure and pulse were not. For the frequency of reactions in blood donors, they quote a 2-5% rate of vasovagal reactions and a 0.08-0.34% of syncope, based on nine previous reports in the literature including the Callahan article.

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Reference #4: Greenbury CL. An analysis of the incidence of "fainting" in 5, 897 unselected blood donors. Br Med J 1942; 1: 253-255.

This study examined the incidence of syncope in blood donors and the influence of age, sex, occupation, lack of food, fatigue, and room temperature on it. A donor was considered to have fainted if any of the fainting-associated symptoms required an interruption of routine. Donors who fainted after leaving the building were not included. 5, 897 donors were randomly chosen (4127 females, 1760 males) from four London sectors. The percent fainting varies among age-grouped males and females from 1.63% to 7.62%. Incidence declined non-uniformly with age in both males and females. In most age groups, females had higher incidence rates. Incidence was higher in clerical workers compared to all other occupation groups. Hours of work (fatigue) and room temperature had no effect. Among female clerical workers, aged 18-25, time from last meal was sigificant, otherwise, it was not.

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Reference #5: Boynton MH, Taylor ES. Complications arising in donor in a mass blood procurement project. Am J Med Sci 1945; 209: 421-436.

This article review the various types complications of blood donation seen among the millions of donors collected by the Blood Donor Service of the American Red Cross from 1941 to 1944. They classify complications into four categories, with group 2 being the one that includes 'faints' (syncope). They list four frequency figures from the literature: 2.8%, 5.5%, 4.2%, and 8.9%. The latter is noted to be simply a subjective report by donors that they suffered an ill-effect. The authors here report data on 2294 'reactors' - the frequency of symptoms within the group, ranging from pallor to loss-of-consciousness-to incontinence, and state that their results are in general agreement with the finding of the British Medical Council report.

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Reference #6: Subcommittee of the Blood Transfusion Research Committee. Fainting in blood donors. Br Med J 1944; 1: 279-286.

In this study, several blood centers in England and Scotland were required to complete a record card for any donor who fainted during a blood collection procedure. A donor was considered to have 'fainted' if they showed signs and symptoms related to fainting which required a delay or change in normal procedure. Multiple symptoms were recorded. The next donor after the symptomatic one, bled by the same person, was recorded also as a control. They report the frequency of recorded faints in two Centers as 5.4% and 5.7% respectively. The effects of various factors on fainting and the incidence of specific symptoms are shown in Tables I - XI.

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Reference #7: Poles FC, Boycott M. Syncope in blood donors. Lancet 1942; 2: 531-535.

This article reports on the experience of the British Army Blood Transfusion Service. Of their first 10,000 donors bled, they record a 2.8% fainting incidence. They define this as any case in which the donor lost consciousness or could not sit or stand without doing so. In one series of 4306 donors, they report 116 faints with a varied time of onset.