Saturday, 7 July 2012

Clinical Biochem: GP Tips

Clinical Biochemistry: GP Tips

With thanks to Prof Jonathan Kay (Consultant Chemical Pathologist, Oxford Radcliffe Hospitals), and the GPs of the
19 Beaumont Street
Journal Club

When should I worry about a low sodium?

► hyponatraemia
·        Increasingly common with more elderly people on diuretics and SSRIs, and many seem to be stable and asymptomatic.  SSRIs are a common cause.
·        Monitor. If patient is stable no need to be too concerned even if sodium is as low as 125.
·        Be aware of rare causes such as Addisons and SIADH
o       Addison’s; suggested by low Na and high K. Ask the lab to do a short synacthen test if clinically suspected
o       SIADH:  if suspect, check urine osomolality; if high this suggests SIADH

And potassium?

►Hypokalaemia
·        K levels go down in warm weather, harmless psueudohopkalaemia; no action needed
·        Worry if K level is < 3
o       Consider referral to exclude Conn’s syndrome if K<3 and no other cause (e.g. diuretics) and hypertensive. Conn’s syndrome is underdiagnosed.
o       In patients on diuretics, if hypokalaemic but K level is >3 there is no evidence this is harmful. If K level is < 3, switch to a k sparing diuretic. This is a better strategy than trying to treat with K supplements

►Hyperkalaemia
·        Usually an artefact due to delay in transportation
·        The rate of change is more important in renal failure, so monitor closely especially if k>5.5 and discuss with renal team if going up or >6

Is there a place for testing magnesium levels?

►Magnesium
o       Relatively cheap test, probably under requested
o       Magnesium deficiency causes symptoms similar to hypocalcaemia e.g. tingling, parasthesia, muscle weakness, leg cramps and tetany
o       Often seen with hypokalaemia and occurs with diuretics, chronic PPI use, GI loss, renal disease, chronic alcoholism and poorly controlled diabetes
o       If <0.7 is associated with QT prolongation
o       Consider testing therefore in, for example, an older person on diuretics with muscle weakness, leg cramps or parasthesia

What about statins and raised CK levels?

►Creatinine Kinase
·        Levels are higher in males and black people of African or Caribbean origin
·        If CK levels are >5 times normal on a statin, you should stop the statin. If less than this and asymptomatic, then no need to worry.

And slightly ‘dodgy’ LFTs?

►LFTS
·        Minor abnormalities very common with fatty liver disease
·        If transaminases < twice the upper limit of normal then further investigation probably not necessary for most patients (especially if clinical picture fits for steatosis)
·        Bilirubin
o       If <1.5 times upper limit of normal, no need to be concerned
o       If largely conjugated (>70%) and <3 times upper limit of normal, then it is likely to be Gilbert’s
o       Consider genetic testing for Gilbert’s. Can request on an EDTA sample requesting ‘genotype for Gilbert’s’ to get definitive diagnosis

Lipid testing: fasting or not, and how often?

►Lipids
·        For risk prediction use total cholesterol/HDL ratio which can be non-fasting (total cholesterol is not affected by eating)
·        With total cholesterol there is a within day variation of up to 25% and also the level tends to rise throughout the day as cholesterol is protein bound
·        Fasting test is required for TGs and LDL.
·        For monitoring, annual tests are enough


Useful sites for further information:

·        www.bettertesting.org.uk  excellent site to have as a ‘favourite’ with good tips on minor abnormalities
·        www.labtestsonline.org.uk     patient info site but very useful


Simon