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
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