Medication and abnormal lab tests

Samir Malhotra

A few months back I had this telephonic conversation Mr. D: I was put on medication for high blood pressure but it is adversely affecting my kidneys.

Me: I remember. I had stressed on the importance of taking medication regularly.

Mr. D: Okay. I am busy now but can we discuss later? 5.30 PM, your office. 

And I was disconnected.

Medications can and do cause adverse effects. One of the several criteria to attribute an adverse effect to a medication is 'temporal relationship' - medication can cause adverse reaction only after it was taken. 

Medications can harm any organ. Blood carries them to all tissues where they interact with cells. But the liver and kidneys bear the brunt as they try to eliminate what we consume. Both these factors increase the possibility of Mr. D suffering an adverse reaction. However, this article is about statistics.

A lab test tells us about the functioning of an organ. When a lab gives a report, typically it also mentions normal reference range -- 0.84-1.21 mg/dL for creatanine. For a test of liver function (AST), it is 5-30 U/L. Values outside the range are considered 'abnormal'. 

The reference range applies to 95 per cent of the normal population. Its implications are important - five per cent of the normal population will have values outside this `normal range'. A value of 32 for AST  would most likely mean that the person is among the remaining five per cent although the report highlights it as 'abnormal'.  If we were to test 1,000 healthy individuals, 50 may have values outside the range for that test. Such people are not necessarily diseased. 

Another possible scenario: the lab report is within the `normal range' but has risen from a previous value. For instance, creatinine value of 0.8 before starting a medication, and going up to 1.2 a few weeks after starting a medication may mean kidney damage, although the value is still within range. 

Now coming back to Mr. D -although not a medico, his observation should not be ignored. High BP itself can damage kidneys, and so can some medication used to treat it (in certain patients). He came with his reports, feeling concerned, says, "See this creatinine report - it's outside the normal range."

Me: How long after starting medication you got this test done?

Mr. D: Two days. He asked me to wait for four weeks but I thought of a test just to be on the safe side. He had my kidneys tested before starting medication and they were normal. I am thinking of stopping the medicine.

I saw his reports - before and after medication. The 'before' report was 0.86 and 'after' 0.78 (`normal range' 0.84-1.21) and the 'after' value was highlighted by the lab! 

First, increased creatinine is an indicator that kidney dysfunction has not decreased; in any case the decrease is too small. Second, though some drugs can produce kidney injury within 24 hours, in this setting two days is too short a period. Third, day-to-day variations in most parameters occur normally. Lastly, differences between labs, within the same lab, sample handling, can all contribute to variations in values. 

My reassurance was 50 per cent successful - he did not discontinue his medication but keeps unnecessarily repeating kidney tests every two weeks - which remain normal.