Ketostix are not always reliable.
Ketostix are a cheap and occasionally accurate way to measure the levels of acetoacetate in urine, one of the three ketone bodies, the other two ketone bodies being acetone and beta-hydroxybutyric acid. The stix are very specific to acetoacetate.
A comparative study of Qualitative tests for ketones in Urine and Serum, Free et al, Clinical Chemistry Vol. 4 Issue 4, 1958
http://www.clinchem.org/content/4/4/323.full.pdf
This is worth repeating, because many people miss it. Ketostix can tell you the levels of acetoacetate in your urine. If you test positive, it does not mean you are in ketosis - it means that this test has detected acetoacetate in your urine. If you test negative it does not mean you are not in ketosis - it means that this test can not detect acetoacetate in your urine.
Inferring that purple on the ketostix means "I am in ketosis" and no purple means "I am not in ketosis" is utterly false. All you know is that this test has shown or failed to show acetoacetate in your urine. You need to know more about the ketone bodies, and about the test, to infer meaning from the ketostix measurement.
The stix only measure the concentration of acetoacetate - drink more water and the ketones become diluted. A very common feature of the ketogenic diet recommendations are for dieters to drink much water, and to take electrolytes in solution. It is my personal experience that thirst rises significantly on a KD, and with it my water consumption. Correspondingly, I pee a lot more now - and that pee is dilute. A reason for many false-negative tests is likely to be due to the diluted nature of a ketogenic dieter's pee.
In principle, the ketostix work because they have segments of sodium nitroprusside - also known as nitroferricyanide. Nitroprusside reacts with acetoacetic acid to turn the segments a shade of colour at a precisely measured duration. Some of the fancier blood measuring strips also include sodium biphosphate as an acid buffer.
The stix must be measured at a precise point in time (15seconds), leave them longer and they will always get darker.
The stix are not sensitive to acetoacetate concentrations less than 50mg/litre.
The stix do not react (much) to acetone, one of the three ketone bodies.
The stix do not react at all to beta-hydroxyburic acid. This is the major cause of a false-negative reading.
It is not just acetoacetic acid which turns the stix purple. Captopril, aspirin, methyldopa, mesna, acetylcsteine, levodopa, methyldopa, phenazopyridine and probably other drugs can also trigger a false-positive ketostix reading.
> Basic Skills in Interpreting Laboratory Data – 2013 by Mary Lee PharmD BCPS FCCP (Editor), http://www.amazon.com/dp/1585283436/
The ketostix are not even very good at diagnosing diabetic ketoacidosis - their designed purpose. The levels of acetoacetic acid in urine can be many times higher than the levels seen in blood. As ketoacidosis subsides, beta-hydroxyburic acid degrades into acetoacetic acid - giving a stronger reading on the stix and a false indication of worsening ketoacidosis.
Recent advances in the monitoring and management of diabetic ketoacidosis, T.M. Wallace, D.R. Matthews DOI: http://dx.doi.org/10.1093/qjmed/hch132
http://qjmed.oxfordjournals.org/content/97/12/773.2
The ADA says:
> RECOMMENDATION: BLOOD KETONE DETERMINATIONS THAT RELY ON THE NITROPRUSSIDE REACTION SHOULD BE USED ONLY AS AN ADJUNCT TO DIAGNOSE DKA AND SHOULD NOT BE USED TO MONITOR DKA TREATMENT.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3114322/#B187
False positives can be seen if the urine is very acidic - after eating lots of vinegar for example. Or if ketones are being produced by intestinal bacteria.
The ketostix degrade on exposure to air - they age badly. They can give false negative readings simply because they have stopped working.
Now, back to nutritional ketosis, the goal of a ketogenic diet. In nutritional ketosis - at least after the initial few days and after a period of keto-adaptation, the majority of ketones produced and cleared are beta-hydroxybutyric acid.
Ketone body kinetics in humans: the effects of insulin-dependent diabetes, obesity, and starvation. http://www.jlr.org/content/25/11/1184.long
There is some indication to suggest that ketotic athletes utilize 100% of free ketones, effectively leaving none for renal clearance:
J Appl Physiol Respir Environ Exerc Physiol. 1978 Jan;44(1):5-11. Changes induced by exercise in rates of turnover and oxidation of ketone bodies in fasting man. http://www.ncbi.nlm.nih.gov/pubmed/627499
Acute nutritional ketosis: implications for exercise performance and metabolism. Cox PJ1, Clarke K2. http://www.ncbi.nlm.nih.gov/pubmed/25379174
The ketostix themselves are not a bad tool. They are just often used inappropriately.
If you know their limitations, and if you are able to mix a number of factors into your reading of the stix beyond just the colour produced (level of hydration, diet over the preceding 12 hours, your own level of keto adaptation etc), then it is possible to reliably get value out of them.
Most new keto dieters won't have the expertise necessary though, and so the recommendation to only use them in the beginning, once or twice, and then go back to concentrating on what is being eaten is really great advice - for most people most of the time.