Editing Reference range (section)

=====Confidence interval of limit=====
The 90% ''confidence interval of a standard reference range limit'' as estimated assuming a normal distribution can be calculated by:<ref>[https://books.google.com/books?id=p7XwAwAAQBAJ&pg=PA65 Page 65] in: {{cite book|title=Tietz Fundamentals of Clinical Chemistry and Molecular Diagnostics|author=Carl A. Burtis, David E. Bruns|edition=7|publisher=Elsevier Health Sciences|year=2014|isbn=9780323292061}}</ref>

: Lower limit of the confidence interval = percentile limit - 2.81 × {{frac|''SD''|{{sqrt|''n''}}}}

: Upper limit of the confidence interval = percentile limit + 2.81 × {{frac|''SD''|{{sqrt|''n''}}}},

where SD is the standard deviation, and n is the number of samples.

Taking the example from the previous section, the number of samples is 12 and the standard deviation is 0.42&nbsp;mmol/L, resulting in:

:''Lower limit of the confidence interval'' of the ''lower limit of the standard reference range'' = 4.4 - 2.81 × {{frac|0.42|{{sqrt|12}}}} ≈ 4.1

:''Upper limit of the confidence interval'' of the ''lower limit of the standard reference range'' = 4.4 + 2.81  × {{frac|0.42|{{sqrt|12}}}} ≈ 4.7

Thus, the lower limit of the reference range can be written as 4.4 (90% CI 4.1–4.7) mmol/L.

Likewise, with similar calculations, the upper limit of the reference range can be written as 6.3 (90% CI 6.0–6.6) mmol/L.

These confidence intervals reflect [[random error]], but do not compensate for [[systematic error]], which in this case can arise from, for example, the reference group not having fasted long enough before blood sampling.

As a comparison, actual reference ranges used clinically for fasting plasma glucose are estimated to have a lower limit of approximately 3.8<ref name=firstaid>Last page of {{cite book |author1=Deepak A. Rao |author2=Le, Tao |author3=Bhushan, Vikas |title=First Aid for the USMLE Step 1 2008 (First Aid for the Usmle Step 1) |publisher=McGraw-Hill Medical |year=2007 |isbn=978-0-07-149868-5 |url-access=registration |url=https://archive.org/details/firstaidforusmle00taol }}</ref> to 4.0,<ref name=uppsala>Reference range list from Uppsala University Hospital ("Laborationslista"). Artnr 40284 Sj74a. Issued on April 22, 2008</ref> and an upper limit of approximately 6.0<ref name=uppsala/> to 6.1.<ref name=Medline-GTT>{{MedlinePlusEncyclopedia|003466|Glucose tolerance test}}</ref>