Are there any important lab tests that will help diagnose pyloric stenosis?

No. Although lab tests can help with the diagnosis, the actual diagnosis is made on history, physical exam and nowadays on ultrasound. Lab tests help determine how severely dehydrated the child is.
Yes. The classic laboratory finding in babies with pyloric stenosis is contraction alkalosis - hypochloremic, hypokalemic metabolic alkalosis. It results from electrolyte losses from continued gastric losses & renal compensatory mechanisms. While this is not pathognomonic for pyloric stenosis, it is an expected finding when this diagnosis is entertained & requires correction prior to operation.
Yes. The most sensitive lab test and noninvasive is a pyloric ultrasound. This simple test will measure the thickness and length of the pyloric muscle. The criteria for pyloric stenosis are quite clear. It is possible for an ultrasound to be normal or equivocal early in the disease and latter on progress to a definitively abnormal study. If symptoms continue and or worsen repeat the ultrasound!
Some help. Pyloric stenosis is generally detected by ultrasound of the pyloric area. An upper gu study is also helpful. These kids have often puked up enough that they loose stomach acid and blood tests show low chloride and decreased acidity. Yet enough puking will give the same results without ps. I depend on my physical exam & the ultrasound.
Ultrasound. Most common test for pyloric stenosis today is ultrasound that shows thickened elongated pyloric muscle. Standard measurements of muscle thickness are well known. A upper gastrointestinal barium study performed in fluoroscopy predated ultrasound and also was quite accurate but involves ionizing radiation.
Yes. There several blood tests as previously listed. An ultrasound can also be used to assess any abnormality of the pyloric muscle.