Superstitious? Don't be.

Its Friday 13th. You have something stressful to do. Do you put it off until another day because it'll be bad luck to do it on Friday 13th? If that stressful thing was a TB test would you think it might be better to arrange for the following week? And if you did, would you be right to do so?

Studies of medicine have a long history of looking to see if there is any temporal pattern to the outcomes of things like operations. You may have seen this on the news recently, a study which said that "patients who are admitted to hospital at the weekend are more likely to die than those admitted during the week". There are lots of other studies that do similar things but with days like Friday 13th, full moons, and superstitious periods found in different cultures and religions.

For example, according to farming folklore, dogs are more aggressive at full moon (Chapman and Morrell, 2000). However, analysis of dog bites requiring admission to hospital during the lunar cycle fails to find any relationship. Other studies also fail to find a relationship between lunar cycles and psychiatric disorders, anxiety, suicide, or emergency hospital admissions (see Raison et al, 1999; Owen et al, 1998; Wilkinson et al,1997; Mathew et al, 1991; Cohen-Mansfield et al, 1989; Thompson and Adams, 1996) but some weak associations with spontaneous full term delivery, aggression and crime (Ghiandoni et al, 1998; Lieber, 1978; Thakur and Sharma, 1984).

By contrast, Yang et al (2008) suggest that in southern China people avoid taking risks during so-called “ghost months” when superstition has it there are wandering ghosts released from hell. The study finds that the number of accidental drownings was significantly lower during ghost month periods. Similarly, another study (O’Reilly and Stevenson, 2000) shows that a superstition over being discharged from maternity on a Saturday leads to higher than expected discharges on Fridays and lower discharges on Saturdays.

The evidence for the links between Friday 13th and human health is mixed. One study (Scanlon et al, 1993) of admissions to accident and emergency on Friday 13th compared to Friday 6th shows that whilst some types of accident (such as poisoning) were not higher on Friday 13th, others specifically transport accidents were higher on five out of 6 friday the 13ths despite there also being fewer vehicles on the road. In fact, the study reveals a 52% increase in the risk of a traffic accident on Friday 13th. Other studies find a link between car accidents and Friday 13th for women but not men (Näyhä, 2002, but later contradicted by Radun and Summala, 2004) and a full moon (Laverty and Kelly, 1998). Friday the 13th is also unrelated to use of emergency services (Lo et al, 2011) blood loss, emergencies or intestinal perforations during operations (Schuld et al, 2011) and post-tonsillectomy haemorrhage (Kumar et al, 2004).

This may all sound like a bit of fun and jokey research: is there much you could do about any of this if it turned out to be true anyway - what is the causal link? Overall, if you lumped all these studies togeher and ran a meta-analysis, you'd probably find no effect.

But anyway, what about studies of animal health? Is there any evidence that Friday 13th is not a good day to be conducting a TB test, for example? Well, Ive looked at the evidence from thousands of TB tests going back to 1992. I compared results of tests read on Friday 13th with those read the day before, on thursday and friday the week before, and on thursday and friday the week after. Realistically, these are the days you would test if you hadnt wanted to read the test on Friday 13th.

For those of you who are superstitious, the results are disappointing. In the analysis, adverse tests (thats finding either a Reactor or IR, or both) on Friday 13th were never more likely to occur than on any of the other possible reading days. If there was any effect, then it was the other way around. The analysis seemed to show that bTB tests the week after Friday 13th turned up more adverse tests than on Friday 13th.

These are only some preliminary results and Ill be writing them up as soon as time allows. But, if you have any other superstitions when it comes to animal health please let me know: email them to me, or tweet them to @garethenticott. I'll also post the full references eventually, but you could probably find them in Google Scholar.

So, if you are testing today, good luck. You may well have been better off not waiting until next week.