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PubMed1992

[Comparison of the effectiveness of Milkofix, a preservative preparation, with traditional preservative agents in the determination of somatic cell count in milk samples using a fluoro-optic-electronic method].

Milkofix (M), a health friendly preservative substance, to be used for milk sample preservation (Trzický, 1990), was compared with other preservatives. Untreated milk samples (N) were tested against samples treated with sodium azide (A; 0.0085 g NaN3 and 0.0630 g NaCl), bronopol (B; 0.0100 g bronopol and 0.090 g NaCl), potassium dichromate (C; 0.0330 g K2Cr2O7 and 0.0670 g KCl) and Milkofix (M; 0.1250 g). The doses of the preservatives A, B, C and M are per 25 ml milk. The somatic cell counts (SB) were determined on a FOSSOMATIC 90 apparatus (FOSS ELECTRIC, DENMARK). In the treated milk samples taken from individual cows the values of SB counts were significantly higher than in N samples if determined within eight hours after sampling (Tab. I): in A higher by 18.6%, B by 26.3%, C by 26.4% and M by 24.3% (P less than 0.05). The significantly higher values of SB counts were recorded in bulk milk samples treated with preservatives in comparison with N samples immediately after sampling: in A by 6.0%, in B, C and M by 12.9% (P less than 0.01; Tab. II). After one-day storage of N samples at a temperature of 4 degrees C these differences are insignificant (P greater than 0.05), thus the results of N, A, B, C and M samples obtained after one-day storage at 4 degrees C can be taken as actual and mutually comparable.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2005

[Sampling and conveyance of mail testing].

In mail testing, a testee collects samples at home from his or her own body, and mails them to the medical institution. Because the testee does not need to attend the medical institution, an advantage of mail testing is that it is not time-consuming. Because the amount of sample is small and time from sampling to measurement is long, the manner of collecting and transporting samples are important factors affecting the precision of the result. There is the risk that inappropriate sample picking can occur, as the testee, who is not skilled in medical testing, may obtain the sample. Moreover, the samples may include blood, urine, secretions, and so on. Depending on the purpose of the test, the transport and sampling method varies, and the character of the sampling device and transport method influence the procedure required. I describe collection methods and other issues in this paper according to medical examination item, sample, and sampling.

Blood Chemical Analysis↗
PubMedDate not supplied

Choosing optimal sampling times for therapeutic drug monitoring.

Recent advances linking pharmacokinetic theory with the selection of optimal sampling times and the influence of sampling times on the interpretation of serum drug concentrations are discussed. For serum drug assay results to be interpreted easily, samples should be obtained during the postdistributive phase (for loading and maintenance doses) or at steady state (for maintenance doses). Beyond this, the choice of sampling times depends on whether peak, trough, or intermediate concentrations are being monitored. When intrapatient and interpatient variation in pharmacokinetic values are considered to be the principal components of variability, the best time for taking a single sample after a test dose in order to predict the average concentration during the steady-state dosage interval is when sample time equals 1.4 times the population half-life of the drug. Optimal sampling times to predict maximum or minimum steady-state concentrations may differ from those used to estimate the average concentration. Efforts to decrease the number of samples needed to estimate accurately steady-state drug concentrations have led investigators to test "single-point" prediction methods following a test dose. However, for drugs administered by continuous infusion, it seems prudent to assay more than one serum sample obtained before steady state is achieved; a single sample obtained too early could contribute to inaccurate predictions. Recommendations are presented for optimal sampling times for commonly monitored drugs.

Blood Specimen Collection↗
PubMed1982

Effect of time of sample collection, milk composition, and stage of lactation on the growth-promoting properties of milk for Streptococcus agalactiae.

Milk samples were taken from noninfected mammary quarters before the AM milking and at intervals after milking. The pH values of the post-AM milk samples were higher than the AM foremilk samples for approximately 3 to 6 hours after milking. When these samples were pasteurized, inoculated with a culture of Streptococcus agalactiae, and incubated at 37 C for 24 hours, there was a greater pH change (delta pH) in the post-AM milks. The higher pH and delta pH values of 3-hour samples coincided with higher concentrations of bicarbonate and chloride, lower concentrations of citrate and phosphate, and lower titratable acidities. Milk samples were taken before the AM milking and 3 to 4 hours later from 2 noninfected quarters of each of 2 cows at weekly intervals for 9 months. The growth of S agalactiae in the milk samples showed that the delta pH and bacterial counts (i) increased as the lactation progressed and milk yield decreased, (ii) were higher in he 3- to 4-hour samples than in the AM samples, and (iii) were higher in the whole milk than in the skim milk of the 3-to 4-hour samples; this difference was attributed to the cream layer and in most instances could be duplicated by layering liquid petrolatum on the surface of skim milk. Also, the bacterial counts in the 3- to 4-hour skim milks were higher than those of the corresponding AM skim milks in 78% of the paired samples, which indicated that another factor, in addition to the cream layer, might be involved in promoting better in vitro growth of S agalactiae in the 3 to 4-hour milk.

Animals↗
PubMed1994

Two-versus three-sample method for estimating gentamicin pharmacokinetic values.

The performances of two-sample and three-sample methods for estimating gentamicin pharmacokinetic values were studied. The medical records of patients who had received a gentamicin dosage consultation at a Veterans Affairs medical center between June 1989 and May 1991 were reviewed. For each patient, the pharmacokinetics service had prospectively used three serum gentamicin concentrations (SGCs) determined from three blood samples taken during an initial gentamicin regimen to estimate gentamicin pharmacokinetic values and determine the alternative would achieve the desired SGCs. In the two-sample method, the authors retrospectively used the initial peak and trough SGCs determined from two blood samples to estimate pharmacokinetic values and establish an alternative regimen. The two methods were evaluated by comparing their results with each other and with the actual pharmacokinetic values determined from SGCs measured during the subsequent gentamicin regimen. A total of 27 patients (all men; mean age, 62 years) were included in the study. The two-sample and three-sample methods differed significantly in their estimates of elimination rate constant and half-life but not clearance, volume of distribution, or the daily dose needed to achieve the intended SGCs. Neither method produced estimates that differed significantly from the actual pharmacokinetic values. The three-sample method was less biased than the two-sample method in the prediction of trough SGCs. In middle-aged and elderly men, a two-sample method and a three-sample method of estimating gentamicin pharmacokinetics differed significantly only in the prediction of trough SGCs. The difference was probably not clinically important.

Aged↗
PubMedDate not supplied

Ochratoxin A in cow's milk and in human milk with corresponding human blood samples.

A method for determining ochratoxin A in milk has been elaborated in which the sample was subjected to a liquid-liquid extraction step and then purified on a silica gel column packed in a Pasteur pipet. The purified samples were analyzed by ion-pair liquid chromatography with fluorescence detection. The detection and quantitation limits for determination of ochratoxin A in cow's milk were 10 and 40 ng ochratoxin A/L milk, respectively. The same limits were valid for the analysis of human milk. A total of 36 cow's milk and 40 human milk samples were analyzed. All samples were collected in Sweden. Ochratoxin A was found in 5 (14%) of the cow's milk samples (range 10-40 ng/mL) and in 23 (58%) of the human milk samples (range 10-40 ng/L). Blood samples were collected from the mothers who gave milk samples. A total of 39 samples were analyzed. All blood samples contained ochratoxin A in concentrations exceeding the quantitation limit (60 ng/L blood). The mean concentration of ochratoxin A in the samples was 167 ng/L blood (range 90-940 ng/L). The concentration of ochratoxin A in human milk was < or = 0.1 of that in the human blood.

Animals↗
PubMed1996

Use of composite milk samples for diagnosis of Staphylococcus aureus mastitis in dairy cattle.

OBJECTIVE: To measure relative sensitivity and relative specificity for use of composite milk samples, compared with that of individual gland milk samples, for diagnosis of Staphylococcus aureus mastitis. ANIMALS: 505 cows suspected of having subclinical mastitis. Of these cows, 172 were considered infected with Sta aureus, based on the results from individual gland samples. PROCEDURE: Composite and individual gland milk samples were collected from cows suspected of having subclinical mastitis, and results of bacteriologic culturing of samples from the same cow were compared. Results were interpreted at the cow level. Relative sensitivity and relative specificity for composite samples were computed from 2 x 2 tables, using results from individual gland samples as references. RESULTS: Relative sensitivity for use of composite milk samples in diagnosing Sta aureus mastitis was 0.63. The relative specificity was 0.98. Factors influencing the relative sensitivity for composite samples were the number of infected glands per cow, the amount of Sta aureus shedding from infected glands, and the proportion of the composite milk obtained from each gland. CLINICAL IMPLICATIONS: Collecting composite instead of individual gland milk samples increases the number of false-negative results in diagnosing Sta aureus mastitis. By collecting consecutive samples from the same cow or by increasing the inoculum volume at culturing, this problem can be diminished.

Animals↗
PubMed1996

Pneumatic transport exacerbates interference of room air contamination in blood gas samples.

OBJECTIVE: To characterize and control the potential interference to P(O2) determinations when blood contaminated with air is sent via a pneumatic tube system (PTS). DESIGN: Both tonometered blood at P(O2)s of 65, 75, 142, and 339 mm Hg and arterial blood gas samples from patients with baseline P(O2)s from 70 to 400 mm Hg were analyzed for P(O2) to determine possible effects of air contamination from PTS transport. SETTING: A large teaching hospital in which a variety of personnel routinely send samples to the laboratory by PTS transport. PATIENTS: Twenty patients under anesthesia for elective surgery and 21 patients in an intensive care unit who had a wide range of P(O2)s. Several additional patients with a preexisting lung pathology likely to cause hypoxemia were selected to provide samples with low P(O2)s. MAIN OUTCOME MEASURES: Measurement of bias in P(O2) between samples sent via PTS and samples walked to the laboratory. RESULTS: Interference from air contamination was worse after PTS transport compared with manual transport of the specimen. Over a wide range, the P(O2) in specimens after PTS transport tended toward 160 mm Hg. Samples from hypoxemic patients were prone to errors in P(O2) that could have resulted in clinical misinterpretation; 5 of 10 samples with a baseline P(O2) less than 85 mm Hg had increases of 10 mm Hg or more when contaminated with air. Cooling samples with high P(O2)s minimized changes to P(O2), probably by increasing the solubility of oxygen. Mechanical buffering by various liners used in the carriers did little to alleviate the interference. Decreasing the speed of pneumatic transport by 50% lessened the effect on P(O2). CONCLUSION: Interference can be minimized by carefully purging samples of all air bubbles using the following protocol: invert syringe to check for air bubbles, then retap and reexpel bubbles if necessary. Personnel that collect and send blood gas samples via PTS should be educated about the problem of interference. Modifications both to pneumatic sample transport systems and to blood gas syringes should be investigated to minimize the effect.

Air↗
PubMed1997

Effects of freezing, thawing, and storage of human liver samples on the microsomal contents and activities of cytochrome P450 enzymes.

Effects of freezing, thawing, and storage at room temperature of human liver samples on the contents and catalytic activities of individual forms of cytochrome P450 (P450 or CYP) were examined. The stability of liver genomic DNA was also investigated. There were no significant decreases in microsomal levels or catalytic activities of P450 enzymes by storage at -80 degrees C for 5 years. We then examined the effects of freezing/thawing of liver samples. Levels of P450 forms were determined immunochemically and by the activities of typical drug oxidation reactions in liver microsomes of human samples, which were divided into two groups. One group of samples was thawed and kept at 25 degrees C for 6 hr and then frozen again and kept for 1 week at -80 degrees C. In the other group, liver microsomes were prepared at the same time (as those from the other group), but not thawed and refrozen. Thawing the liver samples and storage for 6 hr at 25 degrees C decreased contents of total P450 by about 90% and activities of both NADH-ferricyanide and NADPH-cytochrome c reductases by about 80%. However, the decrease in b5 levels was only about 30%. Spectral studies of P450 suggested that thawing the liver samples and holding them at 25 degrees C produced inactive form P420. P450 proteins were detected by immunoblot analysis with or without thawing, but catalytic activities for individual P450s were decreased drastically by thawing and holding samples for 6 hr at 25 degrees C. Only 10% of tolbutamide methyl hydroxylation activity was present, and there was no detectable ethoxyresorufin O-deethylation activity in such microsomes after thawing and holding samples for 6 hr at 25 degrees C. Genomic DNA from human livers was also found to be degraded after the samples were thawing. These results suggested that thawing and holding the liver samples at 25 degrees C decreased the levels and activities of P450s in microsomes and that there are differences in stabilities in individual forms of P450 proteins. P450 proteins determined immunochemically do not always reflect P450-catalytic functions in human liver microsomes because of difficulties in obtaining fresh liver samples.

Cryopreservation↗
PubMed1997

Single-sample methods to measure GFR with technetium-99m-DTPA.

UNLABELLED: Many single-sample methods have been suggested to simplify the methodology of glomerular filtration rate (GFR) measurement. The relative accuracy of these competing methods is still not clear for clinical practice. METHODS: Fifty-four GFR studies with 99mTc-DTPA were performed on 37 adult patients (serum creatinine 0.8-10 mg/dl). Each study included a UV/P, plasma clearance method (three-sample) and single-sample methods. The single-sample methods used were those of Christensen and Groth (modified by Watson), Constable, Dakubu, Groth and Aasted, Jacobsson, Morgan, Russell and Tauxe. RESULTS: When the GFR > or = 30 ml/min (n = 26), all of the single-sample methods were highly correlated with UV/P. The correlation of the single-sample method with the plasma clearance was higher than with UV/P. In this group (GFR > or = 30 ml/min), the Groth 4-hr sample method had the best value of both absolute difference and percent absolute difference (mean +/- s.e. = 11.05 +/- 2.51 ml/min and 14.08% +/- 2.43%, respectively). Most single-sample methods do not perform well at GFR < 30 ml/min (n = 28), and none of them has a good correlation with UV/P or plasma clearance at this level of renal function. However, the Groth and Aasted's 4-hr sample method was the best compared with others (mean +/- s.e. = 8.43 +/- 1.30 ml/min for absolute difference, and 65.91% +/- 16.70% for percent absolute difference). CONCLUSION: Single-sample methods may not correctly predict GFR in advanced renal failure. Groth and Aasted's method with 4-hr plasma sample has both the lowest mean absolute difference and percent absolute difference in both the group with GFR > or = 30 ml/min and GFR < 30 ml/min. All methods perform acceptably at GFR > or = 30 ml/min.

Adult↗
PubMed1997

Evaluation of Triage screening for drugs of abuse in postmortem blood and urine samples.

A Triage screening system that utilizes a simple and rapid deproteinizing procedure using solid sulfosalicylic acid has been evaluated in 62 blood and 27 urine samples obtained from 72 consecutive autopsy cases. Among the blood samples, 11 were positive for amphetamines (AMP), 3 for barbiturates (BAR), and 1 for opiates (OPI). Of the 11 samples that were positive for AMP, only 1 contained methamphetamine (at a concentration of 1.37 micrograms/ml). All 10 samples that were falsely positive for AMP contained phenethylamine, a putrefactive amine, at concentrations of 0.83 to 2,070 micrograms/ml. Apparent negative reactions of the Triage system for benzodiazepines (BZO), AMP, BAR and tricyclic antidepressants (TCA) were observed in three, two one and one blood samples, respectively. These drugs were present in concentrations that were much lower than the lower detection limits of the device. Among the urine samples, nine were positive for AMP, three for BZO, three for BAR, two for OPI and one for TCA. Of the nine samples that were positive for AMP, only three contained methamphetamine (at concentrations of 7.95 to 44.9 micrograms/ml) and six contained no methamphetamine but did contain phenethylamine at concentrations of 0.52 to 14.3 micrograms/ml. False positive Triage reaction were observed only for AMP. There were no false negative Triage reactions from either the blood or urine samples examined for eight classes of drugs of abuse, namely phencyclidine (PCP), BZO, OPI, cocaine metabolites (COC), cannabinoids, AMP, BAR and TCA. The Triage screening system is not able to detect therapeutic or non-toxic levels of drugs of abuse in blood or to discriminate between AMP and putrefactive amines in moderately-to-heavily decomposed blood and urine samples. However, it may still be a useful tool in the field of forensic toxicology for the following reasons: 1) the deproteinizing method causes little reduction in the sensitivity of the Triage screening device to the drugs of abuse, even in turbid urine samples, and 2) it is able to detect toxic levels of PCP, BZO, COC, OPI and BAR in any kind of blood sample.

Amphetamine↗
PubMed1996

Evaluation of a new version of the EnviroAmp Legionella kit for the detection of legionellae in water samples by the polymerase chain reaction.

The detection of Legionella sp. in various types of environmental water samples was evaluated using a revised version of the EnviroAmp Legionella kit (Perkin-Elmer), which is based on the polymerase chain reaction (PCR). The new kit employs a modified protocol for the pretreatment of water samples, which is designed to further reduce the inhibitory action of some samples to the PCR. The results of the new kit were compared with those of culture. Seventy-four water samples were examined. Of these samples, 51 were taken from hospital water systems, 16 were household water samples, 5 were from surface water, and 2 were rain water. These samples were examined both by PCR and by culture on MWY Legionella selective agar. Of the 74 samples, 46 (62%) were positive by both test systems. Twenty-five samples (34%) were positive by PCR, but negative by culture. While 3 samples (4%) were negative by both test methods, no sample was negative by PCR and positive by culture. Statistical analysis revealed high positive predictive values of PCR, when L. pneumophila was detected or when the genus Legionella was detected at high intensity. Negative predictive values were high, when the PCR was negative or only weakly positive for the genus Legionella.

Bacteriological Techniques↗
PubMedDate not supplied

Evaluation of the European four-plate test as a tool for screening antibiotic residues in meat samples from retail outlets.

A modified 4-plate test was used to screen 4795 meat samples from retail outlets in the European Community (EC). This microbial inhibition test uses 3 media seeded with Bacillus subtilis at different pH values (6, 7.2, or 8) and a fourth medium seeded with Micrococcus luteus. Positive samples were confirmed by a receptor test for macrolides, a thin-layer chromatographic method for sulfonamides, or an enzyme-linked immunosorbent assay test for tetracyclines. Inhibition on M. luteus plates, often by beef and veal samples, could not be confirmed. Circumstantial evidence indicated these test results had to be considered presumptively false positives. Of the samples, 95 inhibited at least one plate seeded with B. subtilis. Usually, samples were positive on more than one plate: 70 samples were positive on all 3 plates, and only 6 samples did not inhibit the plate at pH 6. The majority of positive results on plates seeded with B. subtilis, 77 of 89 samples tested, contained tetracycline antibiotics. One sample also contained sulfadimidine. Two other samples contained high levels of enrofloxacine and ciprofloxacine. The 4-plate test is not sensitive enough to detect sulfonamides and quinolones at the EC maximum residue limits, but higher levels may cause inhibition. The 4-plate test can be used to detect meat samples containing tetracycline residues, but the method is too complicated when used only for that purpose.

Animals↗
PubMedDate not supplied

The effect of drug sampling policies on residents' prescribing.

BACKGROUND AND OBJECTIVES: Many clinical educators feel that the availability of drug samples has an influence on the prescribing habits of residents. Therefore, many programs limit the availability of samples. This study compares the prescriptions written in three family practice residency programs with different policies on availability of drug samples. METHODS: We used a prospective observational design to compare three programs of similar size and curriculum but which differed in sampling policies. One program had no limitation of samples, one program limited samples to specifically approved drugs, and one program did not permit drug samples in the clinic. Carbonless duplicate prescriptions were collected and collated over the first 5 months of 1996, and prescribing of nonsteroidal anti-inflammatory drugs (NSAIDs) was monitored. RESULTS: There was a greater percentage of generic prescriptions and use of preferred NSAIDs but no decrease in the "cost per prescription" in the programs that limited or eliminated samples when compared to the program with an "open" sample policy. There was no statistically significant difference between the program with no samples and the program that permitted only approved drugs. CONCLUSIONS: The elimination or control of available drug samples is associated with differences in the prescriptions written by family practice residents, but this effect is fairly small.

Anti-Bacterial Agents↗
PubMed1998

Accuracy of measurements of hemoglobin and potassium in blood samples from peripheral catheters.

OBJECTIVE: To compare measurements of hemoglobin and potassium in blood samples obtained from peripheral venous catheters with measurements in samples obtained via venipuncture. SUBJECTS: 53 adult volunteer patients admitted to a medical ward. METHODS: For each patient, a 10-mL blood sample was obtained from a 20-gauge or larger peripheral venous catheter after a 3-mL volume of fluid was discarded. Immediately afterward, additional samples were obtained by venipuncture, and all samples were sent to the laboratory for measurement of hemoglobin and potassium levels. Separate venipuncture samples were obtained from both extremities in another 10 subjects to assess intratechnique variation. Results were analyzed by using limits-of-agreement analysis. RESULTS: Blood samples were successfully obtained from a peripheral catheter in 32 (60%) of the 53 patients. The success rate was 78% with 18-gauge catheters and 42% with 20-gauge catheters (P = .021). The 95% agreement interval in the set of differences between values for samples obtained via catheter and samples obtained via venipuncture was +/- 0.37 mmol/L for potassium and +/- 7.6 g/L for hemoglobin. The 95% agreement interval for the venipuncture control group was +/- 0.21 mmol/L for potassium and +/- 7.3 g/L for hemoglobin. The preset acceptable interval for potassium measurements was +/- 0.3 mmol/L, and that for hemoglobin measurements was +/- 10 g/L. CONCLUSIONS: Obtaining blood samples from peripheral catheters is an acceptable alternative to venipuncture for hemoglobin measurements but not for potassium measurements. Larger gauge catheters are better than small-gauge catheters for obtaining blood samples.

Adult↗
PubMed2006

Effect of recruitment method and setting on the composition of samples consisting of adult smokers.

OBJECTIVE: This study aims to determine differences in samples of adult smokers recruited by passive or active recruitment in community pharmacy (PP and AP) or passive recruitment in general practice setting (PG), then comparing these samples to an unrecruited cohort of Dutch smokers. METHODS: The three recruited samples were compared on demographics, smoking behaviour, motivational determinants, stage of change and intention to use specific action plans when quitting using multinomial logistic regression analyses; this method was also used to assess whether recruitment affected the acquired samples concerning demographic variables compared with the cohort. PP, AP and PG response rates were compared with Tukey post hoc tests. RESULTS: Significant differences were found for both AP and PG smokers, compared with PP smokers. Most important results include a higher rate of pre-contemplators in the AP sample and a lower educational level in the PG sample. All recruited samples appeared to be significantly different from the cohort sample. CONCLUSION: Recruitment method and primary care setting does influence the sample recruited, this should be taken into consideration when recruiting a specific sub-sample. PRACTICE IMPLICATIONS: These results are valuable for researchers contemplating recruitment in a primary care setting.

Adult↗
PubMed2006

Operational feasibility of lot quality assurance sampling (LQAS) as a tool in routine process monitoring of filariasis control programmes.

Lot quality assurance sampling (LQAS) with two-stage sampling plan was applied for rapid monitoring of coverage after every round of mass drug administration (MDA). A Primary Health Centre (PHC) consisting of 29 villages in Thiruvannamalai district, Tamil Nadu was selected as the study area. Two threshold levels of coverage were used: threshold A (maximum: 60%; minimum: 40%) and threshold B (maximum: 80%; minimum: 60%). Based on these thresholds, one sampling plan each for A and B was derived with the necessary sample size and the number of allowable defectives (i.e. defectives mean those who have not received the drug). Using data generated through simple random sampling (SRSI) of 1,750 individuals in the study area, LQAS was validated with the above two sampling plans for its diagnostic and field applicability. Simultaneously, a household survey (SRSH) was conducted for validation and cost-effectiveness analysis. Based on SRSH survey, the estimated coverage was 93.5% (CI: 91.7-95.3%). LQAS with threshold A revealed that by sampling a maximum of 14 individuals and by allowing four defectives, the coverage was >or=60% in >90% of villages at the first stage. Similarly, with threshold B by sampling a maximum of nine individuals and by allowing four defectives, the coverage was >or=80% in >90% of villages at the first stage. These analyses suggest that the sampling plan (14,4,52,25) of threshold A may be adopted in MDA to assess if a minimum coverage of 60% has been achieved. However, to achieve the goal of elimination, the sampling plan (9, 4, 42, 29) of threshold B can identify villages in which the coverage is <80% so that remedial measures can be taken. Cost-effectiveness analysis showed that both options of LQAS are more cost-effective than SRSH to detect a village with a given level of coverage. The cost per village was US dollars 76.18 under SRSH. The cost of LQAS was US dollars 65.81 and 55.63 per village for thresholds A and B respectively. The total financial cost of classifying a village correctly with the given threshold level of LQAS could be reduced by 14% and 26% of the cost of conventional SRSH method.

Adolescent↗
PubMed1997

An examination of methods for sample size recalculation during an experiment.

In designing experiments, investigators frequently can specify an important effect that they wish to detect with high power, without the ability to provide an equally certain assessment of the variance of the response. If the experiment is designed based on a guess of the variance, an under-powered study may result. To remedy this problem, there have been several procedures proposed that obtain estimates of the variance from the data as they accrue and then recalculate the sample size accordingly. One class of procedures is fully sequential in that it assesses after each response whether the current sample size yields the desired power based on the current estimate of the variance. This approach is efficient, but it is not practical or advisable in many situations. Another class of procedures involves only two or three stages of sampling and recalculates the sample size based on the observed variance at designated times, perhaps coinciding with interim efficacy analyses. The two-stage approach can result in substantial oversampling, but it is feasible in many situations, whereas the three-stage approach corrects the problem of oversampling, but is less feasible. We propose a procedure that aims to combine the advantages of both the fully sequential and the two-stage approaches. This quasi-sequential procedure involves only two stages of sampling and it applies to the stopping rule from the fully sequential procedure to data beyond the initial sample which we obtain via multiple imputation. We show through simulations that when the initial sample size is substantially less than the correct sample size, the mean squared error of the final sample size calculated from the quasi-sequential procedure can be considerably less than that from the two-stage procedure. We compare the distributions of these recalculated sample sizes and discuss our findings for alternative procedures, as well.

Anti-HIV Agents↗