I recommend to use Dempster-Shafer or kalman fusion. Both can be used to fuse different measurements. 

Combining several studies sounds like a meta analysis problem, although I'm not very familiar with this area.

Perhaps you could do a Bayesian analysis on the first data set, and then update the result with the second data set, using the posterior as the prior in the second Bayesian analysis.

http://labstats.net/articles/combining_experiments.html

Thanks both for helping out. I have been trying to grasp the Depster-Shafer and kalman fusion, but this seems a bit out of my league - and most of the literature on these techniques is on differences in questionnaire categories which is not fully applicable to TSH (i.e. a skewed continuous measurement).

Reading on Wouter's suggestion, this would make sense if I wanted to pool the results - however, I will have individual participant data available and wish to first pool the datasets and then analyze the data. Nontheless, even if I would do a two-step analysis: If the measurements are dissimilar between the cohorts, I will misclassify individuals when pooling - regardless of how I pool. I need to overcome this misclassification, as it will bias my effect estimates towards the null.

Hi Tim,

Your assumptions sound very logical to me.   If test 'A' has a 10th percentile value, then that should mean that 10 percent of cases (in the population used to normalize the test originally) fall below that value and 90 percent above.   Test 'B' should be the same: the 10th percentile is the 10th percentile. 

With that said, test 'A' and test 'B' might not give the same result for the same patient. There are certainly differences between laboratory tests --- the same patient may test in the 'high' range at one laboratory using one test, and test 'normal' in another (or the same) laboratory using a different test.   Why this is so isn't clear; but from what I understand there is a lot of lab-to-lab inconsistency---which suggests variation among technicians---as well as variations among the original normative populations that each company's tests were normalized on when they were first developed. 

But this is not necessarily a bad thing.  If company 'A' and company 'B' used different normative populations to initially validate their tests, these may represent legitimate patient populations (or healthy populations) to compare an individual patient against.  

I would say go ahead and use all five tests, but convert the patients' test scores to Z-scores (also known as standard scores) so they will be comparable to one another: i.e., a test value of +1.6 z-units on test 'A' will be assumed to be the same as +1.6 z-units on test 'B'.  (note of course that you must use the mean and SD of the normative population the tests were originally validated on by the company, and not your own sample's mean and SD! ---a mistake some students make.  Thus your entire sample may fall below the tests's mean value, for instance).   

In work I have been involved with, we've used a variety of intelligence and development tests to assess patient populations with in this way.  Most intellectual/developmental tests have a mean of 100 and an SD of 15 but they each measure different things, so we simply reported these scores in z-units (for example "group X scored on average 1.2 standard deviations lower on all five developmental tests than group Y").   It sounds like the goal of your study isn't to validate one TSH test against another;  rather, you wish to assume that they ARE all equal (normally distributed and easily converted to standard scores (z-scores)). You should probably try to verify the distributions of scores that were used to normalize each test as it was developed, to be sure they are all from a similar distribution (hopefully normal) so that you can use the same z-score formula and thus compare apples to apples. . 

Pooling the scores might have some risk because of the above-mentioned inconsistences between tests (which may boil down to differences between the populations they were normalized on).  If one company's tests are very different from all the rest (perhaps because they used a diseased population to normalize their test, and the other companies used healthy people) then this could wash out any differences that are real.  But that information might not be available.  You might want to report your results in the way we did, e.g., 'four out of five tests for the female subgroup showed TSH z-scores at least one unit lower than in the male subgroup" or similar language.   You should probably also  report results in a table which somehow breaks down the results of each company's test.  Some lab test results are notoriously unreliable; I don't know about TSH tests but clinician readers might give more weight to some over others. They'll want to see which tests might be reporting consistently higher or lower TSH values even if you also pool the results..   You will probably have to state some conclusions and/or limitations regarding the comparability of the five tests, which again isn't your goal per se. 

I['m not sure how you're analyzing this but you might have a cohort effect, a lab effect, and a test-type effect to deal with. 

Another thought is that you could correct or adjust one test's norms to comply with the rest; in regard to your example of one test's 5th percentile being equal to another test's 10th percentile.  I think I've seen some literature on this. But it opens another can of worms and may be tangential to your readers' ultimate interests.   Good  luck! 

Thanks for your response Jerry. The Z scores seem to work alright so far, but still I feel that the misclassification is causing regression to the null.

With regards to the normalisation of the manufacturer, the thing is we have much better data than the manufacturers. In addition, they manufacturers do not normalize their measurements based on another test, but rather on solutions with a certain fixed amount of TSH. As such, it is difficult to normalize based on the validation procedures used by the manufacturer.

Although there are some papers describing the correlation between assays, this does not allow for adequate standardisation given that it is likely that the difference between assays for lower concentrations is dissimilar to that of higher concentrations.

One thing I have been thinking of is taking out some of the between-population variation that is caused by differences in TSH determinants. For example, ethnicity is a known TSH determinant and the populations differ in ethnicities. As such, standardisation to ethnicity in each cohort may take out some of that variation. 

In addition, it may also prove valuable to find a way to adjust for differences in kurtosis and skewness between cohorts, taking the largest cohort as a reference for example. Unfortunately, I have not yet managed to find any such techniques.

Hi Rudolf, thanks for thinking along.

I am afraid I do not understand your concerns. TSH is the exposure variable in our case, so the fact that it has a skewed distribution is very unlikely to lead to skewed residuals (i.e we an use it to study the effects of interest and will meet the normality assumption).

We do not have an issue with reference ranges, but with comparability of the measurements. In the Katayev study the labs performed before hand "Standardization to the same methods, reagent lot numbers, calibrators, controls, and standard operating procedures significantly reduces the between-laboratory variability in test results.". What they fixed beforehand, is exactly what we need to fix retrospectively with our data - but I am having trouble with the how.